If you want to improve on Surprise Attack's damage the best advise is to remove it from your bar and replace it with Wrecking Blow. Only half kidding....

Anyways, onto the fun since improving Surprise Attack isn't going to help much. I never used Reapers. For PvP sure but never thought it much benefit in PvE. I've seen others mention it's because mobs don't have much armor to begin with but who knows. Never used ambush either since it was hard enough managing magicka with all the Disguise, Surprise Attack spam.

Let's say you're attacking a 3 pack. It's very easy(and tedious). You fight two and ignore the 3rd. Once the 1st is dead you mix the 3rd into the rotation. You just use Surprise Attack and Funnel Health pretty much until you have a target <25% health. So crouch, SA 1st, FH 1st, disguise. Either SA the 1st again if needed or move to the 2nd and SA that one, KB 1st, FH 2nd. Now it's a 2 pack. Do the same thing again. Disguise and SA/FH your primary target until you can execute and mix in the last target and repeat ad nauseum.

Should work easily enough for VR7. Somewhere along the way you'll realize the mobs have too much health for your magicka pool and Siphoning Attacks will get a permanent home on your bar. Initial Surprise Attack will do around 1,750 with Siphoning Attacks on so expect 2-3 stabs at VR10 per target. Yes, you can start with Siphoning Attacks off and toggle as needed but, again at some point, reducing tedium will be more important than anything else in your world. Needless to say you'll have to pepper in light attacks to refill resources.

I know it's a huge pain but at VR9-10 you're going to want to Disguise every few seconds to keep the Shadow Barrier passive up. Things hit HARD so you'll either want to be invis and fluffy or vis and, well, not hard but a little less fluffy.

Anyways, bars at VR10 were:

1--Surprise Attack, Funnel Health, Killer's Blade, Siphoning Attacks, Shadowy Disguise...Soul Harvest for ultimate-gain boost only

2--Volcanic Rune, Refreshing Path, 3 spots for leveling skills...Veil of Blades for main ultimate

VR and RP were only used for when things went pear shaped. After a while though I just learned to accept death and didn't even bother. But you can pop a rune, drop a path and scoot out of trouble most of the time.

This was all with resto staff. If you must have DW it should work just fine. Add Blood Craze as a stam dump until Siphoning Attacks bumps it(or Funnel) off. You really only need one HoT as most fights you'll likely only get hit a few times.

At VR10 I hear you have to have Begging on your main bar but I'd ditched my NB at that point so can't say for sure.

ETA: sorry to be so long winded.

The truth no one wants to hear .. use a resto staff. You do everything you mentioned with a resto staff and passives +10% damage. Even in Craglorn I open up in sneak to Surprise Attack -> off balance > Heavy Attack -> Impale .. they are dead or one impale from dead. Cloak -> surprise -> heavy they are on teh ground again. Usually around then I am trying to get a swallow soul on at least one for the extra healing. If you can just kill them from there go for it if not you can always cloak -> suprise -> heavy and have one of them out of the picture for a few seconds.

Also if you have trouble getting the fight started in sneak if you are in sneak and use cloak to get around them it still does the huge initial damage as long as that is how you start combat.

Personally I don't bother with Mark or Ambush. Also Mass Hysteria if you are having trouble with the other two. Start off your fight getting the one down, while it is down pop the fear the other two go running until the third is dead and back in to cloak to hit the next one.

Thanks you all the responses and advice.

After having So many Problems with Vr7 mobs, I've had to change my skills around a little, I dropped ambush in favour of Immovable, I rarely use Reapers any more, as the lowering of My armour is just too much now I hit Vr7 i swapped it out for Swallow soul. Ultimate is Soul Harvest. I have also changed to DW Swords with Sharpened trait.

Initial Surprise attack damage from Stealth/crouch is now up to 2483. Still not enough to 1 shot, but Hey, I can use Killers blade to finish off and get a nice heal.

At least that's one mob down before I get smooshed. Sometimes i can get 2 down if Im lucky, But never 3, not now Im fighting Vr7+ mobs.

Might change back to DW daggers as the DW swords seems to be a little Glitchy, the Animation will go off but don't always get the damage, I'll be swinging away (yeah coz I ran out of Magicka lol) and even though the animation is going off im not always getting the damage each swing Unlike with DW daggers.

My Second/off set is Resto staff, I use Siphoning attacks, Sap Essence, Inner light, Combat prayer, and Quick syphon, Veil of blades as Ultimate. this works Really well as long as I have my Ultimate up, But with the Ultimate Generation Nerf yesterday, its much harder to be able to use my ultimate all the time.