UPDATE: EVERYTHING HERE REFLECTS THE STATE OF PTS 3.1.4, AS IT WAS COMPLETED BEFORE HOTR WENT LIVE WITH ADDITIONAL CHANGES TO CRIT BONUSES/PRECISE/THIEF/SHADOW. THEREFORE THESE NUMBERS DO NOT REFLECT A TRUE COMPARISON BETWEEN PREHOTR AND HOTR LIVE, BUT PREHOTR AND HOTR 3.1.4.
HOWEVER...
THE COMPARATIVE CALCULATIONS TO NECRO AND SPINNERS REMAIN UNCHANGED, WHILE JULIANOS LOSES 2.1% CRIT ON ITS SET BONUSES, REDUCING ITS EFFECTIVENESS SLIGHTLY.
SINCE ALL BUILDS USED PRECISE / THIEF, THE OVERALL DPS POTENTIAL OF EACH BUILD DROPPED EQUALLY ACROSS ALL OF THEM AND SO DOES NOT EFFECT MY CALCULATIONS.

Unlike my previous post which was a narrowly focused comparison of many sets' bonuses against an arbitrary baseline, here I will try to specifically look at the differences between just three sets in a more realistic build environment. While the benefit of this approach will be providing a more true to life representation, it will also be less general. Players who do not match the parameters of the build I use will differ to a greater or lesser degrees in their own experience from what I present. Additionally, the results here will be entirely mathematically derived, based on the UESP Build Editor. While the editor approximates ingame values extremely well, anamolies can still exist which skew results slightly. Actual ingame testing will always yield the most accurate results. So take this for what it is.

SUMMARY STATEMENTS (TLDR):
I am writing this after coming to the end of the post, having written it sequentially, in an attempt to briefly sum up my findings. I still strongly encourage going through the whole post though. I will bold the most important parts.
1. The overriding result seems to be that whatever differences there are, they are small and will not make a sizeable impact on the average player. The sets are closer here than in my initial analysis, and most differences occur in a rather small range.
2. Consequently, running any of these sets will achieve similar results, especially if you work to synergize the rest of your build well. The difference between pet strength and weapon attack power essentially cancel each other out, making it unnecessary to think you have to run Necro to run pets. Of course, if you are going for a full summoner build then your total pet power will likely overtake your weapon attack deficit, but not by a wide margin.
3. Julianos is good all around in PvE.
4. Spinner's is good particularly when you are not close to the penetration cap, but still can perform well even when reaching it. Be mindful, though, Gilliamtherogue believes there are underlying ingame factors not represented in the math that work against Spinner's, leading it to parse a little lower than the math would make one expect. It is still a very good set, but will likely not outpace other sets as much as the mathematics would lead one to believe.
5. Necro provides a sizable shield buff compared to any other set, which still makes it strong in PvP despite the nerf. The other factors commonly mentioned such as more resources for casting, pet strength and weapon attack scaling all end up having values too small to really be worth making arguments for or against in terms of how they should impact the set's standing. Since Necro essentially remained the same moving into HotR while other sets got buffed, it has lost some of its standing in PvE and is now unlikely to consistently outparse other builds like Julianos/Mother's Sorrow. Personally, I think leaving the set alone would have left it truly competitive with these other setup choices, but the difference is so small it's certainly still open for debate. More ingame testing by the community will help refine our collective understanding.

NOTE: BECAUSE THIS POST WAS WRITTEN BEFORE HORNS OF THE REACH, ALL REFERENCES TO LIVE REFER TO PREHOTR
These are the sets we will be examining, along with their LIVE variants:
Necro: Necropotence as it is planned to be in HotR
1096 × 3 = 3288 + 3150 = 6438 Magicka
Necro (no nerf): Necropotence in HotR if they did not nerf the 5piece bonus
3288 + 4000 = 7288 Magicka
Julianos: Julianos considered as the sole 5piece set.
1096 Magicka, 299 Spell Damage, 1066 × 2 = 2132 ÷ 219 = 9.7% Crit Chance
Julianos (2%): Julianos considered as one of two paired 5piece light/crafted armor sets. To demonstrate the operational benefit of this set over a second 5piece light armor set we will add an additional 2% Magicka as compared to the other sets which would only benefit from a 4% Magicka increase for a 61 setup vs Julianos' ability to achieve the full 6% bonus.
1096 Magicka, 299 Spell Damage, 9.7% Crit Chance, 2% more magicka
Spinner's PvE: Spinner's as it will appear in HotR with its 5piece nerfed to 3450 Penetration, against PvE targets.
1096 × 2 = 2192 Magicka, 129 Spell Damage, 3450 ÷ 50000 = 6.9% PvE Penetration
(More discussion on this percent later.)
Spinner's PvP: Spinner's as it will appear in HotR with its 5piece nerfed to 3450 Penetration, against PvP targets.
1096 × 2 = 2192 Magicka, 129 Spell Damage, 3450 ÷ 66000 = 5.2% PvP Penetration
The only adjustment we will be making for Spinner's PvP will be a 1.7% reduction of Penetration to account for the higher "level" of players as opposed to npcs. We will not be adjusting any of the baseline gear or skills, which will be setup for a PvE environment, mainly because this would bring in far too many corrupting variables to our basic set comparison. Consequently, the base stats will be slightly higher in this case than is probable with a true PvP build.

So here we go...
Math that gets us to our first chart:
Baseline Build Characteristics:
 Max level/CP Altmer Magicka Sorc with 64 attribute points in Magicka
 2 Monster pieces med/heavy with 1piece Magicka bonus, 5 ______, 4 Moondancer, all Legendary, 7 Divines, all magicka/damage enchants,
 Precise staff, Thief mundus: We are choosing these so that the switch from Live to HotR won't effect our values at all, since 7 + 11 = 18 (Live) and 9 + 9 = 18 (HotR)
 Major Sorcery, Major Prophey active via potions
 Major Breach applied by an outside source
 Blue Health/Magicka food
 CP: 56 Ele Expert, 50 Erosion, 54 Elfborn, 75 Thaumaturge, 5 MaA = 230 in Blue tree
 Active skill bar: Crystal Fragments, Force Pulse, Volatile Familiar, Inner Light, Empowered Ward, Thunderous Rage
Bonuses relevant to Max Magicka:
 Altmer passive (10%)
 Inner Light with the Mage's Guild passive (7%)
 511 Undaunted Mettle (6%)
 CP 300 (24.6%) < 13 × 0.2 = 4.6% + 20%
10 + 7 + 6 = 13 + 24.6 = 47.6%
So we have 47.6% boosted Max Magicka
Necro LIVE: 50127 Magicka, 2502 SD, 65.8% Crit, 67% Crit Dmg, 12% CP Magic Done, 92% Pen, 5% Damage Done
11346
Prepping HotR calculation:
5 Magicka bonuses increased from 967 to 1096 (3 from Necro, 1 from monster, 1 from Moondancer)
967 × 5 = 4835
1096 × 5 = 5480
5480  4835 = 645 more magicka in HotR from 24 piece bonuses
4000  3150 = 850 less magicka from Necro 5piece
645  850 = 205
50127 ÷ 1.476 = 33961
33961  205 = 33756
33756 × 1.476 = 49823
50127 (Live)  49823 (HotR) = 303 less Magicka in HotR than Live with Necro
Necro HotR: 49823 Magicka, 2502 SD, 65.8% Crit, 67% Crit Dmg, 12% CP Magic Done, 92% Pen, 5% Damage Done
11302 (0.39% worse than Live)
Prepping HotR Necro (no nerf) calculation:
5 magicka bonuses = 645 more magicka
33961 + 645 = 34606
34606 × 1.476 = 51078
51078  50127 = 951 more magicka for HotR Necro (no nerf) than Live Necro
51078  49823 = 1255 less magicka for HotR Necro than HotR Necro (no nerf)
Necro (no nerf) HotR: 51078 Magicka, 2502 SD, 65.8% Crit, 67% Crit Dmg, 12% CP Magic Done, 92% Pen, 5% Damage Done
11489 (1.26% better than Live)
Julianos LIVE: 42352 Magicka, 2879 SD, 72.1% Crit, 67% Crit Dmg, 12% CP Magic Done, 92% Pen, 5% Damage Done
11095
47.6% + 2% + 0.4% from CP contribution (which is 0.2% for every 1% added) = 50%
42352 ÷ 1.476 = 28693
28693 x 1.50 = 43039
Julianos (2%) LIVE: 43039 Magicka, 2879 SD, 72.1% Crit, 67% Crit Dmg, 12% CP Magic Done, 92% Pen, 5% Damage Done
11202
Prepping HotR Julianos calculation:
3 Magicka bonuses increased (1 from Julianos, 1 from monster, 1 from Moondancer)
967 x 3 = 2901
1096 x 3 = 3288
3288  2901 = 387
42352 ÷ 1.476 = 28693
28693 + 387 = 29080
29080 x 1.476 = 42922
42922  42352 = 570 more magicka for Julianos HotR than Julianos LIVE
2 Crit bonuses increased
688 x 2 = 1376
1066 x 2 = 2132
2132  1376 = 756
756 / 219 = 3.45% more crit for Julianos HotR than Julianos LIVE
Juliano HotR: 42922 Magicka, 2879 SD, 75.5% Crit, 67% Crit Dmg, 12% CP Magic Done, 92% Pen, 5% Damage Done
11357 (2.36% better than Live)
42922 ÷ 1.476 = 29080
29080 x 1.50 = 43620
Julianos (2%) HotR: 43620 Magicka, 2879 SD, 75.5% Crit, 67% Crit Dmg, 12% CP Magic Done, 92% Pen, 5% Damage Done
11465 (2.34% better than Live)
4000 ÷ 50000 = 8%
Spinner's PvE LIVE: 43779 Magicka, 2664 SD, 65.8% Crit, 67% Crit Dmg, 12% CP Magic Done, 100% Pen, 5% Damage Done
11576
4000 ÷ 66000 = 6%
Spinner's PvP LIVE: 43779 Magicka, 2664 SD, 65.8% Crit, 67% Crit Dmg, 12% CP Magic Done, 98% Pen, 5% Damage Done
11345
Prepping HotR Spinner calculation:
4 Magicka bonuses increased (2 from Spinner, 1 from monster, 1 from Moondancer)
967 x 4 = 3868
1096 x 4 = 4384
4384  3868 = 516
43779 ÷ 1.476 = 29660
29660 + 516 = 30176
30176 x 1.476 = 44539
44539  43779 = 760 more magicka for Spinner's HotR than Spinner's LIVE
8  6.9 = 1.1% less PvE Penetration for Spinner's HotR than Spinner's LIVE
6  5.2 = 0.8% less PvP Penetration for Spinner's HotR than Spinner's LIVE
Spinner's PvE HotR: 44539 Magicka, 2664 SD, 65.8% Crit, 67% Crit Dmg, 12% CP Magic Done, 98.9% Pen, 5% Damage Done
11571 (0.04% weaker than Live)
Spinner's PvP HotR: 44539 Magicka, 2664 SD, 65.8% Crit, 67% Crit Dmg, 12% CP Magic Done, 97.2% Pen, 5% Damage Done
11373 (0.2% weaker than Live)
So here is the chart with all the values we've collected. I will subtract 10000 from each value to better highlight their differences.
So we can see how each set impacts our Effective Spell Power, but we should also consider several other aspects of these sets in terms of their additional benefits and drawbacks to get a more full picture of how they compare.

JULIANOS: This is the most straightforward set of the three we are looking at. There are no conditions to be met, no circumstances where it behaves differently. The only extra thing to consider is what we have already discussed, namely the ability to finish a 511 setup due to this set being crafted. The Julianos (2%) represents this benefit, but keep in mind that this benefit is voided when comparing 5/4/2 setups. It is only relevant in a 5/5/1 setup where the other set is a dropped light armor set, when compared to the option of running two 5 piece light armor sets together.
 One point to consider regarding PvP is that the two crit bonuses on this set will be mitigated to a degree due to shields not being critable and players using Impen to reduce crit damage.

SPINNER'S: The question of whether Spinner's will perform to its full potential all comes down to how much additional penetration you can rely on.
 In PvE, we are trying to hit 18200 without going over, once all sources of penetration are considered. In the case above, I am assuming an environment where the only additional source of penetration other than myself is a tank/healer applying Major Breach. If you are running vMA and can't or don't want to slot Ele Drain, then it will be very unlikely that you will hit the penetration cap. If you are in a trial with Major Breach being applied, and also a tank applying Infused + Toruge's Crusher enchant, and an offtank wearing Alkosh, the chances are good you will hit the penetration cap easily without Spinner's. So consider the content you are doing and how much penetration you can rely on without Spinner's. If 3450 more would not put you over 18200 then you will get the full value out of it, which will grant you a little under 7% more damage.
 Diminishing Returns: In fact, 7% is the low end of what Spinner's can provide. If you were wearing only Spinner's with no other sources of penetration, it's impact would be closer to 17% rather than 7%. Even still, it's worth getting as close as you can to the penetration cap, but it does mean that for lower CP players, Spinner's should perform even better.
 In PvP, two factors need to be consideres. First, players have a wide variety of resistance levels. A light armor player may only have 1011k magicka resistance you have to get through, while a heavy armor player might have 2030k. Second, damage shields do not have any innate resistances, so your Spinner's 5th piece is useless against them. It may help you burst down a shielding opponent after breaking through the shield or if they don't refresh it (like from being CC'd), but you will do less damage to the target when their shield is up than if you were using some other set that increased spell damage or max magicka. When it does hit a nonshielded target, the 5th piece amounts to about 5% more damage.

NECROPOTENCE: Now to our main focus for this thread. What additional benefits and drawbacks come in to play for Necro in PvE and PvP that should have a bearing on how it compares to the other sets we are considering.

BENEFITS:

 More max magicka means
MORE CASTS:
To compare Necropotence to Julianos and Spinner's, let's consider an average skill cost of 3000 magicka. This will make our cast potential divisible by 3000, so the relevant max magicka thresholds will be 42k, 48k, and 51k. We will assume the skill we are using is instant cast, and so we can cast it once every second.
Julianos and Spinner's: 42000 ÷ 3000 = 14 casts/seconds before depletion
Necro: 48000 ÷ 3000 = 16 casts/seconds before depletion
Necro (no nerf): 51000 ÷ 3000 = 17 casts/seconds before depletion
Now, it's important to understand that a higher max magicka pool doesn't provide the SAME benefit as more magicka recovery. Two players that have 30k Magicka left in their pool and 798 magicka recovery (which is what all of these builds have) will regenerate and continue to spend magicka at equal rates. There is no added benefit to more max magicka on regen in that context.
However, it does have a SPECIFIC benefit. Namely, that it takes longer to run out of resources initially compared to a player with lower resources. That extra time of free casting equates to some extra UPFRONT resource regeneration.
Rounding up all of our builds' magicka regen to 800 from 796 for simplicity, and knowing that magicka recovery is applied every two seconds, we can divide this number by 2 to examine how much magicka regen all of these builds have innately per second.
800 ÷ 2 = 400 magicka regen per seconds
Leaving Necro (no nerf) for our final consideration, we'll compare Necro to Julianos and Spinner's first, which puts our magicka ceiling at 48k. Now, for a player with fully depleted resources and 400 regen, it would take them 120 seconds (2 minutes) to allow their magicka recovery to fully restore that magicka pool without using any other tools for regen like heavy attacks, potions, poisons, etc.
48000 ÷ 400 = 120
Now we know that Julianos and Spinners' would take 14 casts/seconds to deplete their resource pool. During that 14 seconds, the player would still be regenerating magicka, which means it would take them 14 seconds less to reach 48k. We want to compare hitting the same ceiling as Necro to see the difference between them, and it is possible for Julianos and Spinner's to regenerate that much magicka even though their pool is smaller because the player will be continuing to spend magicka as they generate it and so will not stop regenerating due to having hit their own resource ceiling.
120  14 = 106
48000 ÷ 106 = 452.83 x 2 = 905 effective magicka recovery
To confirm this, how much time would it take for a player starting at 0 magicka to get to 48000 magicka with 452.83 regen per second?
48000 ÷ 452.83 = 106 seconds
So over roughly the first 2 minutes of combat, a player with 42k magicka has an effective magicka recovery of 905, which would then drop back down to 800 until the player is able to fully replenish their resource pool again.
905  800 = 105 additional effective magicka recovery in the first 2 minutes of combat
Applying the same process to
Necro:
120  16 = 104
48000 ÷ 104 = 461.53 x 2 = 923
923  800 =
123 additional effective magicka recovery in the first 2 minutes of combat
123  105 =
18 more effective magicka recovery in the first 2 minutes of combat with Necro vs. Julianos and Spinner's.
Now let's compare against Necro (no nerf). For this, we need to recalculate Julianos and Spinner's to the higher 51k ceiling:
51000 ÷ 400 = 127.5
127.5  14 = 113.5
Julianos and Spinner's: 51000 ÷ 113.5 = 449.34 x 2 = 898
898  800 =
98 additional effective recovery in the first 2 minutes of combat (slightly longer than the previous calculation, we are just using a broad time increment here)
127.5  17 = 110.5
Necro (no nerf): 51000 ÷ 110.5 = 461.53 x 2 = 923
923  800 =
123 additional effective recovery in the the first 2 minutes of combat
923  898 = 25 more effective magicka recovery in the first 2 minutes of combat with Necro (no nerf) vs. Julianos and Spinner's.
127.5  16 = 111.5
Necro: 51000 ÷ 111.5 = 457.34 x 2 = 914
914  800 =
114 additional effective recovery in the first 2 minutes of combat
923  914 = 9 more effective magicka recovery in the first 2 minutes of combat with Necro (no nerf) vs. Necro.
Comparing the difference between Necro (nerf)'s boost over Necro as well as Julianos and Spinner's, it appears that every extra cast available equates to about 89 more magicka recovery over roughly the first 2 minutes of combat.
To complete the above calculations, we focused only on the key resource values that would have been divisible by our average 3000 magicka cost skill. Having any more magicka before hitting the next divisible threshold wouldn't make any difference. Of course in real play, you will use skills with various values, so even castable values smaller than a full integer can provide a small boost. With that in mind, here are the true values for dividing each setup's max magicka by 3000:
Necro LIVE: 50127 Magicka / 3000 = 16.706
Necro HotR: 49823 Magicka / 3000 = 16.607
Necro (no nerf) HotR: 51078 Magicka / 3000 = 17.026
Julianos LIVE: 42352 Magicka / 3000 = 14.117
Julianos (2%) LIVE: 43039 Magicka / 3000 = 14.346
Juliano HotR: 42922 Magicka / 3000 = 14.307
Julianos HotR (2%): 43620 Magicka / 3000 = 14.54
Spinner's PvE/PvP LIVE: 43779 Magicka / 3000 = 14.593
Spinner's PvE/PvP HotR: 44539 Magicka / 3000 = 14.846
Necro in HotR will lose 0.1 to its castable value, so remains essentially unchanged. Necro's benefit over Julianos and Spinner's only amounts to about 18 more effective magicka recovery over roughly the first 2 minutes of combat. Considering all of these sets have a magicka recovery of 798, this amounts to a 2.25% temporary boost to magicka recovery for Necro.
If Necro (no nerf) is considered instead, it gains 0.3 castable value, enough to move into the next divisible zone for our average 3000 magicka skill. This results in a gain of about 9 more effective magicka recovery over roughly 2 minutes when compared to Necro Live and Necro HotR. Against Julianos and Spinner's it moves to about 25 more effective magicka recovery over the first 2 minutes of combat. This amounts to a 3.13% temporary boost to magicka recovery for Necro (no buff). So the percentage difference increase for Necro (no buff) over Necro would only be 0.88%.
CONCLUSION:
The question of whether additional magicka for casting should weigh against Necro's ESP position depends on whether you think that 18 or 25 more effective magicka recovery over roughly the first 2 minutes of combat warrants any kind of adjustment. To me, the extra benefit of 2 or 3 more casts at the beginning of a fight is so small that it should have no meaningful bearing on Necro's ESP position.

 More max magicka means
BIGGER SHIELDS:
For this comparison, we will examine the tooltip values of Hardened Ward (including the 33% increase) and Harness Magicka. Since the primary concern about shield size is PvPbased and has no significant bearing on PvE, we will be looking at stacking these two commonly used shields on magicka Sorcerer. To view the impact that each set has on shield size, we will compare each build to a baseline where no 5piece set is active. Also, to simulate a PvP environment, we will half the shield sizes. Keep in mind the rest of our gear is PvE based, and so the actual values of the shields we get may be different from a true PvP build using different complimentary sets. This is not relevant to our comparison, though, as we are only concerned with how much bigger the shields are from the baseline, which will be entirely what the set we are looking at provides.
First, lets look at LIVE values, except for Julianos (2%):
Baseline LIVE: 36210 Magicka, 16036 HW, 15141 HM
16036 x 1.33 = 21327 ÷ 2 = 10663
15141 ÷ 2 = 7570
10663 + 7570 =
18233
Necro LIVE: 50127 Magicka, 22200 HW, 20960 HM
22200 x 1.33 = 29526 ÷ 2 = 14763
20960 ÷ 2 = 10480
14763 + 10480 = 25243
25243  18233 =
7010 bigger shield stack than Baseline LIVE
Julianos LIVE: 42352 Magicka, 18757 HW, 17709 HM
18757 x 1.33 = 24946 ÷ 2 = 12473
17709 ÷ 2 = 8853
12473 + 8853 = 21326
21326  18233 =
3093 bigger shield stack that Baseline LIVE
Spinner's LIVE: 43779 Magicka, 19388 HW, 18306 HM
19388 x 1.33 = 25786 ÷ 2 = 12893
18306 ÷ 2 = 9153
12893 + 9153 = 22046
22046  18233 =
3813 bigger shield stack than Baseline LIVE
Now let's see if we can derive a consistent ratio of max magicka to total shield stack size:
36210 / 18233 = 1.9859595
50127 / 25243 = 1.9857782
42352 / 21326 = 1.9859326
43779 / 22046 = 1.9858024
So using a ratio of 1.9858 will work for us. We will simply divide each max magicka value by 1.9858 to get the total shield stack size of each build.
Julianos (2%) LIVE: 43039 Magicka / 1.9858 = 21673
21673  18233 =
3440 bigger shield stack than Baseline LIVE
Prepping HotR calculation:
2 Magicka bonuses increased from 967 to 1096 (1 from monster, 1 from Moondancer)
967 × 2 = 1934
1096 × 2 = 2192
2192  1934 = 258 more magicka in HotR
36210 ÷ 1.476 = 24532
24532 + 258 = 24790
24790 × 1.476 = 36590
Baseline: 36590 Magicka / 1.9858 =
18425
Necro: 49823 Magicka / 1.9858 = 25089
(36.168% more than Baseline)
25089  18425 =
6664 bigger shield stack than Baseline
Necro (no nerf): 51078 Magicka / 1.9858 = 25721
(39.598% more than Baseline)
25721  18425 =
7296 bigger shield stack than Baseline
Julianos: 42922 Magicka / 1.9858 = 21614
(17.308% more than Baseline)
21614  18425 =
3189 bigger shield stack than Baseline
Julianos (2%): 43620 Magicka / 1.9858 = 21965
(19.213% more than Baseline)
21965  18425 =
3540 bigger shield stack than Baseline
Spinner's: 43779 Magicka / 1.9858 = 22046
(19.652% more than Baseline)
22046  18425 =
3621 bigger shield stack than Baseline
The following chart shows the shield sizes over their respective Baseline values:
The difference from Necro (no nerf) to Julianos is:
7296  3189 = 4107
25721 / 21614 =
19% bigger
The difference from Necro to Julianos is:
6664  3189 = 3475
25089 / 21614 =
16% bigger
The difference from Necro to Julianos (2%) is:
6664  3540 = 3043
25089 / 21965 =
14.222% bigger
The difference from Necro to Spinner's is:
6664  3621 = 3043
25089 / 22046 =
13.8% bigger
The amount of shield stack reduced by the nerf to Necropotence is:
7296  6664 = 632
25721 / 25089 =
2.519% smaller
CONCLUSION:
Since the size of damage shields is tied to max magicka and no other value, it is not surprising that Necropotence outpaces Julianos and Spinner's by a rather wide margin. Even with the nerf, Necro still provides 36% increased shield size over our Baseline and 16% over Julianos. We notice that even with the nerf to the 5th piece bonus, we only lose 2.519% of our shield stack size, so if the point of the nerf was to tone down shield stacking, it really doesn't do much to accomplish that. At the same time, remember that shield size is largely irreverent in PvE. Even the 15k Harness Magicka from our Baseline LIVE build would be sufficient for 99% of all PvE content. So from a PvE perspective, it seems that the improved shield size we see from Necro should not have a bearing on its ESP. From a PvP perspective, there is a clear defensive advantage with running Necro, which the nerf did little to mitigate. And the fundamental question remains: "Should one set provide 1316% bigger shields over other sets and still provide the same damage output?" At least in PvP, the honest answer is clearly "no." So some negative positioning of Necro seems appropriate to account for this. The unfortunate side effect is that PvE ESP positioning also gets effected, even though larger shield size doesn't warrant a reduction in that environment.
SUGGESTION:
It seems to me that if the dev team does feel that shield size remains a problem, even after applying the 50% reduction in PvP some time ago, then another strategy other than nerfing max magicka sets is warranted. Either create a Major/Minor buff category for shields, so that things like Hardened Ward and Harness Magicka will overwrite one another with the same Major Shielding buff rather than stack, or else apply a diminishing returns factor to shield stacking that reduces the size of shields applied based on how much shielding you currently have applied. Either of these solutions would satisfy any issues that Necro poses in PvP and allow it to be tuned specifically for DPS to stay competitive and even BiS by a respectable margin for any petbased build.

 More max magicka means
STRONGER PETS:
Similar to how we approached shields, we will examine our build without any 5piece set as our Baseline, and then note the changes in Volatile Familiar Damage Pulse tooltip strength on each build. Because Crit and Penetration both effect pet strength but do not appear on the tooltip, we will need multiply our tooltip values by those to get a more accurate result. Again, we will do the LIVE versions first, except Julianos (2%) and see if we can derive a ratio to max magicka. Note that we will NOT be adding in any "attacker bonus" (CP, damage done, damage taken). These would be the same across all builds and so would needlessly inflate our numbers. Consequently, the damage numbers listed here will be smaller than ingame, but should still be the same in relationship to one another.
Baseline LIVE: 36210 Magicka, 3698 Pulse
3698 x 1.44 = 5325 x 0.92 =
4899
Necro LIVE: 50127 Magicka, 5118 Pulse
5118 x 1.44 = 7369 x 0.92 = 6779
6779 / 4899 =
38.6% increase over Baseline Live
Julianos LIVE: 42352 Magicka, 4324 Pulse
4324 x 1.483 = 6412 x 0.92 = 5899
5899 / 4899 =
20.4% increase over Baseline Live
Spinner's PvE LIVE: 43779 Magicka, 4470 Pulse
4470 x 1.44 = 6436 x 1.00 = 6436
6436 / 4899 =
31.3% increase over Baseline Live
Spinner's PvP LIVE: 43779 Magicka, 4470 Pulse
4470 x 1.44 = 6436 x 0.98 = 6307
6307 / 4899 =
28.7% increase over Baseline Live
36210 / 3698 = 9.7817793
50127 / 5118 = 9.7942555
42352 / 4324 = 9.7946345
43779 / 4470 = 9.7939595
We can go with 9.794 for this one.
Julianos (2%) LIVE: 43039 Magicka / 9.794 = 4394 Pulse
4394 x 1.483 = 6516 x 0.92 = 5994
5994 / 4899 =
22.6% increase over Baseline Live
Baseline: 36590 Magicka / 9.794 = 3735 Pulse
3735 x 1.44 = 5378 x 0.92 =
4947
Necro: 49823 Magicka / 9.794 = 5087 Pulse
5087 x 1.44 = 7325 x 0.92 = 6739
6739 / 4947 =
36.2% increase over Baseline
Necro (no nerf): 51078 Magicka / 9.794 = 5215 Pulse
5215 x 1.44 = 7509 x 0.92 = 6908
6908 / 4947 =
39.6% increase over Baseline
Julianos: 42922 Magicka / 9.794 = 4382 Pulse
4382 x 1.5 = 6573 x 0.92 = 6047
6047 / 4947 =
22.2% increase over Baseline
Julianos (2%): 43620 Magicka / 9.794 = 4453 Pulse
4453 x 1.5 = 6679 x 0.92 = 6144
6144 / 4947 =
24.2% increase over Baseline
Spinner's PvE: 43779 Magicka / 9.794 = 4469 Pulse
4469 x 1.44 = 6435 x 0.989 = 6364
6364 / 4947 =
28.6% increase over Baseline
Spinner's PvP: 43779 Magicka / 9.794 = 4469 Pulse
4469 x 1.44 = 6435 x 0.972 = 6254
6254 / 4947 =
26.4% increase over Baseline
The following chart lists all percentage increases over the Baseline for pet damage:
So comparing Necro to the next best option here, Spinner's PvE, you gain about 7.6% pet damage from Necro or 11% with Necro (no nerf).
On the bottom end, you gain 14% over Julianos with Necro and 17.4% with Necro (no nerf).
So we have a range of about 717%, or 714% not considering Necro (no nerf) going into HotR.
In a Pet Sorcerer parse, the Familiar Damage Pulse can often account for 1213% of your total dps. The Familiar's basic attack is very weak, amounting to 1% or less of your total dps.
Let's first look at the LIVE versions of Julianos and Necropotence to see what changes moving into HotR. Julianos LIVE is missing out on 18.2% extra damage from the Familiar. And we'll say the Familiar accounts for 14% of our total DPS. In a 38000 parse we would get:
38000 x 0.14 = 5320 dps
38.6  20.4 = 18.2%
5320 x 0.182 = 968.24 dps
38968 / 38000 =
2.547% total dps loss from Julianos LIVE over Necro LIVE, all else being equal.
In the best case LIVE scenario,
Necro LIVE vs. Spinner's PvE LIVE:
38.6  28.7 = 9.9%
5320 x 0.099 = 526.68
38526 / 38000 =
1.384% total loss from Spinner's PvE LIVE
And looking at
Necro LIVE vs. Julianos (2%) LIVE:
38.6  22.6 = 16%
5320 x 0.16 = 851.2
38851 / 38000 =
2.239% total loss from Julianos (2%) LIVE
Now let's take
Necro (no nerf) compared to Julianos in HotR, we have 17.4% lost dps.
39.6  22.2 = 17.4%
5320 x 0.174 = 925.68 dps
38925 / 38000 =
2.434% total dps loss from Julianos
Necro compared to Julianos:
36.2  22.2 = 14%
5320 x 0.14 = 744
38744 / 38000 =
1.957% total dps loss from Julianos
Necro compared to Julianos (2%)
36.2 x 24.2 = 12%
5320 x 0.12 = 638.4
38638 / 38000 =
1.679% total dps loss from Julianos
Necro compared to Spinner's PvE
36.2  28.6 = 7.6%
5320 x 0.076 = 404.32
38404 / 38000 =
1.063% total dps loss from Spinner's
Comparing the difference between Necro LIVE and Necro:
38.6  36.2 =
2.4% loss in HotR
Comparing Necro LIVE and Necro (no nerf)
39.6  38.6 =
1% gain in HotR
CONCLUSION:
Considering that the move from LIVE to HotR amounts to about a 2.4% drop in total dps from the Familiar weakening, while sets like Julianos (2%) and Spinner's PvE close the gap to within 11.6% total dps less than Necro, one of the biggest negative impacts of the Necro nerf is seen here. Indeed, the gap wasn't amazingly wide even with LIVE, but the buffs to 24 piece bonuses combined with the Necro nerf amout to a noticable loss of distinctivness for Necro. Yes, it will still do the most pet damage, but not by much, leaving other sets' ability to outpace the loss with more dps on nonpet skills. If Necro had not been nerfed, it would have gained 1% more total dps, which would have helped keep pace at least somewhat with the buffs other set received.

DRAWBACKS:


PET REQUIRED for 5th piece bonus:
The set requires a living combat pet for its 5th piece bonus to apply. This means that a player wanting to run a build without a pet will not see a benefit to this set over pretty much any other 5piece set. This limits Sorcerers moreso than Wardens and Nightblades since their pets require 2 skill slots, while the Warden/Nightblade can easily keep this set active with the Betty Netch skill or the Shades skill, which cannot be killed and only require one bar slot.
PVE: In PvE, the question is, "does slotting a pet enable enough dps to be worth it?" For Sorcerers and Wardens, the answer is a pretty strong yes, as the Volatile Familiar and Betty Netch are both strong skills worthy of finding a home on a PvE player's bar, even without Necro. For Nightblades, this is less so, as their Shades skill is primarily defensive, though for content like vMA can be quite handy. So if keeping a strong pet skill on your bars isn't much of a hinderance, how is this a drawback?
1. Sorcerer pets require two skill slots, so you're giving up more utility for the pet than other classea have to.
2. Sorcerer pets don't have great AI and can get lost/stuck or killed by standing in red or by oneshot mechanics, forcing the caster to lose time resummoning.
 Neither of these issues effect Wardens using Netch (but do apply if just using the Warden's bear) or Nightblades using Shades. So in those cases, if the player was planning to run those skills anyway, this is not a drawback for them.
PVP: In PvP, a "meta" nonpet magicka Sorc won't be able to run this set because they simply do not have the bar space to accommodate either the Familiar of the Matriarch. If they do chose to run a pet build, they will be sacrificing some of the standard utility of PvP magicka sorcs to do so. Wardens and Nightblades won't feel as significant a cost unless they were not planning on running the skills that enable Necro anyway.
CONCLUSION:
Although pets can be a hinderance in some contexts for Sorcerers, in general they are not, and don't strongly demand a positive adjustment to Necro's position. If this set only worked with Sorcerer pets, perhaps it would be another matter, but we have to consider the other classes that are able to use the set as well with less difficulty. Due to the nonmathematical assesment of this drawback, it remains largely subjective what sort of impact, if any, it should have on ESP.


WEAPONS ATTACKS SCALE BADLY with max magicka
Because weapon attacks scale at a rate of about 10.5:1 with Spell Damage, just like abilities, but only scale at about 40:1 with Max Magicka, unlike abilities, sets that focus predominantly on max magicka bonuses, such as Necro, will suffer from overall weaker weapon light and heavy attacks than sets that boost spell damage, crit, or penetrarion.
Weapon attacks usually make up about 1520% of a dps parse. This is what each of the builds list as their respective values in Live and HotR:
In a previous 78 second parse using a pet sorc build I got:
42 Light Attacks
7 Heavy Attacks
A rate of 6:1.
...which makes sense since we are doing an 8 second rotation with one HA that lasts 2 seconds.
So we will do: ((LA x 6) + HA) × ((Crit Chance x Crit Damage) + 1) x Penetration
The values listed here are taken directly from the UESP Build Editor and represent what would be actual ingame damage.
Necro LIVE: LA = 2619 HA = 11853
2619 x 6 = 15714+ 11853 = 27567
0.658 x 0.67 = 0.44 + 1 = 1.44
27567 x 1.44 = 39696 x 0.92 =
36520
Necro HotR: LA = 2613 HA = 11826
2613 x 6 = 15678 + 11826 = 27504
0.658 x 0.67 = 0.44 + 1 = 1.44
27504 x 1.44 = 39605 x 0.92 =
36436 (0.2% worse than Live)
Necro (no nerf) HotR: LA = 2636 HA = 11928
2636 x 6 = 15816 + 11928 = 27744
0.658 x 0.67 = 0.44 + 1 = 1.44
27744 x 1.44 = 39951 x 0.92 =
36755 (0.6% better than Live)
Julianos LIVE: LA = 2745 HA = 12425
2745 x 6 = 16470 + 12425 = 28895
0.658 x 0.67 = 0.44 + 1 = 1.44
28895 x 1.44 = 41608 x 0.92 =
38279
Julianos (2%) LIVE: LA = 2758 HA = 12484
2758 x 6 = 16548 + 12484 = 29032
0.658 x 0.67 = 0.44 + 1 = 1.44
29032 x 1.44 = 41806 x 0.92 =
38461
Julianos HotR: LA = 2757 HA = 12477
2757 x 6 = 16542 + 12477 = 29019
0.755 x 0.67 = 0.505 + 1 = 1.505
28895 x 1.505 = 43486 x 0.92 =
40007 (4.5% better than Live)
Julianos (2%) HotR: LA = 2768 HA = 12530
2768 x 6 = 16608 + 12530 = 29138
0.755 x 0.67 = 0.505 + 1 = 1.505
29138 x 1.505 = 43852 x 0.92 =
40343 (4.9% better than Live)
Spinner's PvE LIVE: LA = 2621 HA = 11863
2621 x 6 = 15726 + 11863 = 27589
0.658 x 0.67 = 0.44 + 1 = 1.44
27589 x 1.44 = 39728 x 1 =
39728
Spinner's PvP LIVE: LA = 2621 HA = 11863
2621 x 6 = 15726 + 11863 = 27589
0.658 x 0.67 = 0.44 + 1 = 1.44
27589 x 1.44 = 39728 x 0.98 =
38933
Spinner's PvE HotR: LA = 2636 HA = 11928
2636 x 6 = 15816 + 11928 = 27744
0.658 x 0.67 = 0.44 + 1 = 1.44
27744 x 1.44 = 39951 x 0.989 =
39511 (0.5% worse than Live)
Spinner's PvP HotR: LA = 2636 HA = 11928
2636 x 6 = 15816 + 11928 = 27744
0.658 x 0.67 = 0.44 + 1 = 1.44
27744 x 1.44 = 39951 x 0.972=
38832 (0.02% worse than Live)
30000 has been removed from each value to show the difference between results more clearly.
Julianos (2%) vs Necro
40343 / 36432 =
10.735% more weapon damage with Julianos (2%) than Necro
In a 38000 dps parse, if weapon attacks made up 20% of the total dps (which they did in the 78 second parse I mentioned above), they would be worth 7600 dps. Out of that, the difference between Necro and Julainos would amount to about 10% of 7600, or 760 dps. So, all else being equal, if Necro parsed for 38000, Julianos would parse for 38760.
38760 / 38000 =
2% more total dps from weapon attacks with Julianos (2%) than Necro
CONCLUSION:
Whatever gains Necro makes with pet damage seem to be counteracted very closely by its weak weapon attack scaling, so much so that it is likely that builds that are strong with weapon attacks and nonpet skills will be able to overtake any pet damage deficit.

Thanks for reading! I know it was a lot. I basically used this post like a workbook, figuring out the data as I went. If I made any errors, please feel free to call them out and I will do my best to correct them.
Remember that while all the values I used were realistic, they were made within a specific build context. The info here is meant to provide a broad view of how the various topics covered interact with each other. Actual ingame dps testing will always be able to give the most accurate results, so I encourage those who are interested in this info to get in game, test, and share your own findings so we can all continue to understand, appreciate and dominate ESO together.