I would recommend that you stick with a 17" wheel up front. If you go up to an 18", you would either have to go up in overall diameter, or get a lower profile tire, like a 265/35/18. A shorter sidewall in the front than the rear might upset the balanced handling and degrade the ride. 18" tires are more expensive too. A set of Eagle SC's will run about $1100. It's about $200 more to run four 18" BFG G-Force. Take a look at tirerack.com for other brands that fit the z06 wheels. Also, I don't mean to be picky, but "series" usualy refers to the sidewall aspect ratio, not the width. Good luck with your new wheels.
-liquidRubber
-liquidRubber