C5 Z06 with stock wheels ("Alcoa style" 275s on 17/9.5 front and 295s on 18/10.5 rear)
I've had the car for almost two years now and I'm slowly getting close to what it can do in stock form (suspension/power) on summer street tires (continental extreme contact 340TW rating)
At first I was intimidated by it on the track but I finally decided to keep it (for as long as possible)

As I get closer to the limits, the first step will be getting better tires (something in the 200TW range)
Second step would be trying to fit wider tires (I think 315 will fit on the rear with the stock setup)
On the front though ... the 17s limits my tire choices to 285 (Nittos I think) so I started considering getting another set of identical rims (I know, it might take me a long time to find a set but I'm not in a rush).

The question is: without rolling the fenders or cutting the plastics, on stock suspension and stock ride height (even if I want to, I can't lower it because even now I'm scraping the front plastic dam going up and down my driveway and that won't change for the foreseeable future), on the track (at the limit in the corners) can I fit the rear wheels on the front without rubbing? If yes, can I use 315s or should I stay with 295s?

Or, another question is, if I install the rear wheels on the front in my garage (I can do that now), what clearances should I look for around the tire so it won't rub at full compression?

 

At stock ride height you can fit almost anything in the front. I use stock rear wheels on all 4 corners with 285/35 tires as my regular street driving setup. I am lowered and they do not rub anything. Again at stock ride height a 315 on that wheel will fit fine.

However, your reasoning for not lowering isnt practical. Those rubber air dam pieces are wear items. I replace mine almost every year from tracking. They rub all the time. Doesnt hurt anything.

 

Not lowering because it rubs when I get on/off the driveway is like a mental block for me. That noise bothers me like crazy. I get the same thing in the carousel on Shenandoah (Summit Point motorsport park).
Even the manual says they're wear and tear items but I still can't deal with that (yet).
I guess, with time, they'll wear more and more and/or I'll stop worrying about it

 

Eventually they'll self clearance and stop making noise.

This is an 18x10.5" with a 295/35/18 lowered on stock bolts. I believe camber was only -.4*, so once I bump that I expect it to tuck, at least more than now, at static height. I've not bottomed this thing out, I wonder if the camber gain alone isn't enough to keep it from hitting the lip?

Update: I just checked the alignment on the car and that wheel has -1.2* of camber.

 

Resurrecting an old thread with some updates:
I ran two events on the square setup (285/35R18), Hankook RS4s
First one was me going relatively slow on a slow track (not a lot of info or data), second one was on Watkins Glen (more important for this thread).
Being my first time there I started slow but, as the sessions progressed, I started pushing more and more.
Conclusion? GM engineers were right: you need more rubber on the back. At least on fast tracks.
First, air pressures (numbers aren't important, differences are): starting with 27PSI on all four, at the end of the sessions, I would get to 35 on the rear and 32 on the front.
Second, behavior: relatively easy to induce (controlled) oversteer exiting the turns, even with low throttle inputs, NEVER experienced understeer (I could have, had I started to push harder but then the problem of oversteer on a really fast track would have been ... you know what I mean)

Otherwise, no rubs or any clearance issues, 285 (or even 295) are ok on the front, 315 on the rear should be just as fine.

I believe that, FOR MY CAR (and me as a driver), I need wider tires on the rear (305s or 315s).

For tighter tracks and/or autocross ... the square setup might be best. I might find out in the future.