like in OB some spells will only effect nastys below a certain level. the highest leveled spell is 25. for game play are we to assume that a 25 level spell will efect all nasties that aren't imune to it?

No, you need to get augment perks or dual casting to increase the effective spell level.

Look here for more explanation:

http://uesp.net/wiki/Skyrim:Illusion

Note that animage and kindredmage don't apply to undead even if you have the mastermind perk.

Illusion is pretty much an all or nothing school. Either you invest in every single perk and maximize your skill, or not...and then find yourself lacking in that particular school of magic. Unlike other some other skills, I don't believe Illusion has a single perk that could be considered a throwaway perk.