I've done a bit of research on it myself, and that's without perks.Kopikatsu said:I wouldn't recommend an illusion build. At level 35 or so (Forget the exact level), EVERY SINGLE enemy becomes immune to all illusion spells. Basically, it's a waste of time.
The level-increase perks add 22 levels onto the 25-cap of the master-level illusion spells (allowing you to affect level 47 targets), and apparently dual casting doubles that level cap on top of that.