This may sound crazy but try calling up TireRack and asking them if they'll fit. They seem to be extremely knowledgeable about these issues.
I wish I could answer your question but I'm not going to pretend that I can. At the very least, I gave you a bump.
BTW, they'll look awesome. I've seen Z06 rims on C4's and they look totally awesome. Don't know if they were Z06 size but they were the Corvette Z06 style and I couldn't stop staring.