as i recall the level cap for nastys was 25 for oblivion. according to wiki some nastys cap at around level 60 such for dragons. illussion spells effectiveness is capped. are there many nastys that won't be effected by illusion spells?

Dual-casting effectively doubles the level of the spell, over and above any increase from the other perks.

Dragons are immune to illusion.

I think with all the illusion perks that increase the effectiveness, the fact that they are cumulative (Animage and Rage Stack for e.g), the fact you can duel cast, you can affect things up to level 90+ which is pretty much everything in game that is susceptible. Then vampires have 25% stronger illusion if you wanted to go even further.