The low proc chance is extremely depressing and will need to be buffed if this set is to be competitive with other gear.
So lets take a magicka sorc, a class that will probably benefit most from this set. lets even assume that you are able to average 2 attacks per second due to weaving. And lets take a 60 second fight.
60 second fight with 2 attacks per second gives us 120 attacks total. The proc chance of Scathing mage depends on crit chance. We have:
10% Base
10% Light Armor Passive
12 % Precision
10% Inner Light
6.8% Scathing mage
Lets even assume that you are running thief stone (although I would run Atronach with 2 piece Kena and full spell damage jewelry enchants)
18.3 % Full thief
Total Crit chance: 67.1%
This means that out of 120 attacks 67.1% will crit, or 80.52 attacks. Scathing Mage has a proc chance of of 10%, which means that 8.052 attacks will cause scathing mage procs. Lets us also assume that the RNG gods are good and that those proccing attacks do not intersect, meaning each proc will give you exactly 6 seconds of the buff. 8.052 x 6 = 48.312 secconds of scathing mage, which is 516 spell damage at the legendary level. So in a 60 second fight you will have gained 516 x 48.312 seconds of the proc, or 24928.992 spell damage( or 415 per second). If we replace Scathing mage with Overwhelming 4 piece and 1 Torug's we will get 129 x 3 spell damage = 387 spell damage. 387 damage over 60 seconds is 23220 spell damage. The difference between the two is 1708 spell damage divided over 60 seconds is 28.4832 spell damage per second. So with averraging 120 attacks per 60 minute fight (impossible, especially if you're proccing crystal frags), thief stone up, with perfect trait Scathing mage (Good luck farming those perfect traits), AND with the RNG gods making sure that none of those procs intersect you will gain an average of 28.4832 spell damage and 6.8% crit chance. Now how likely are you to get those attacks to line up perfectly where you do not lose procs to intersects? It becomes increasingly lackluster if you think about it.
Personally I would not run thief and would run Atronach instead to offset Kena's cost, while maintaining spell damage enchants on all my gear. Without thief the situation becomes even more grim. 120 attacks x 48.8% crit chance = 58.56 attacks or 5.856 attacks that will proc scathing mage. 5.856 x 6 = 35.136 seconds. 35.136 seconds x 516 = 18130.176 (Or 302 spell damage per second). 18130.176 is much less than the 23220 that we would get with overwhelming and torugs. Furthermore if you ran the new Julianos set, you would keep the crit and would only lose 3 spell power per second and you wouldnt have to grind out for traits, since it is craftable...