Title says it all. Im thinking of trying this interesting hybrid build of Sneaking + Conjuration while grabbing silent casting and muffle sound from illusion. The idea came to mind that getting Illusion as a tertiary perk tree might actually be even better then getting the mage armor perks in my case, to throw additional chaos in there, but I have been looking at master spells and they only work on targets that are 25 or lower.

How much higher does the perk let you use illusion spells? They would be sort of useless if they stopped working completely a third of the way to the lvl cap o.O

No one?

Another question is, what IS your opponents actual level? If you are level 60, who of your enemies will be 60, 65 or 30 or 15? Who knows. But that's also something that needs to be looked into.