I'm planning on going for an illusionist mage with perks added for extra backstabbing damage. Does anyone know if I will need to be good at sneaking for this (= putting 5 perks into stealth) or if my illusion spells will be compatible with backstabbing?

Well, yeah Sneak is important. You will need to be able to successfully Sneak up on people without them noticing you.

Sneak has all the backstab skills - it also has a hide in plain sight skill which I imagine would synergise EXTREMELY well with illusion. (Fear -> Hide -> Enemy is now a long way from your hiding place giving you time to sneak about for a better position)