I don't think anyone found an optimal setup yet.
My setup looks like this (Slaanesh, lvl4, I can hide him in a fast unit, only another lvl2 with him):
Third Eye of Tzeentch
Dispel Scroll
Power stone
Spell Familiar
Enchanted shield
Dispel Scroll
If I added more sorcerers, I would drop one DS and get Collar of Khorne and I would use him in a Knight unit.
Since everything needs to be in synergy in an army list, let me give you advices (which you can totally disregard if you wish).
Standard magic item I would be considering for any of your sorcerers:
- Power Familiar, Book of Secrets, Infernal Puppet, Spell Familiar (for your lvl4).
- 1 dispel scroll (no more no less)
Other magic item not often used but to seriously consider:
- Blood of Tzeentch (for casting on 2-3 dice, check the stats I made:
http://www.librarium-online.com/for...haos-tactica-work-progress-6.html#post1444699). Not sure about that one but you could also try to use the Blood of Tzeentch after your enemy dispel attempt.
- Skull of Katam (not a good idea if you want my opinion, it's an
all in the same basket idea).
- Power stones to drain early dispel scrolls. Do it at the end of the magic phase.
You will not like the idea, but did you considered a lvl4 Slaanesh sorcerer. I field one in my magic heavy army (and I faced daemons and empire with it). I love the lore, especially the Ecstatic Seizures spell which in my opinion is better than Infernal Gateway on average (no ward save and no regeneration save on that one [add to the fact that daemons infantry are T3 most of the time and you have quite a deadly spell against them]). I use more or less the same tactic than with a Tzeentch sorcerer, I cast small spells first (buboes, missile or hysterical frenzy) and then attempt Ecstatic Seizure on 5 dice (sometimes 4).
Anyway, a lvl2 Slaanesh sorcerer could be a nice addition to your council. If you give him the Book of Secrets, he should be able to have access to at least 2 missile attacks. You can also mount him on a Slaanesh Steed with bloodcurdling roar and use him in a Slaanesh marauder horsemen unit.
On a side note, I would still prepare myself for loosing the magic domination (plan for the worst, hope for the best...). So maybe add a Collar of Khorne (ward save at same time) and/or maybe a Fury of the Blood God both on sorcerers inside your main units could be nice.
As for your units I would do something similar to the one made by Pinkus for his DP: all cavalry to keep it fast and zippy.