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255/325 is the max width according to Michelin's tire sizing guide for PS4S and Cup 2's. I spent months deciding on factory size or up sizing.
I went with factory size due to it being a daily and wanting to keep the run flats(3 nails so far) and with the paragon spacers the car has enough poke. I have a non Z51 car but a concern with a Z51 car is PTM and the ELSD are calibrated specifically for the factory size tire and compound, although going non run flat or Cup 2's or any other track day tire run into the same issue.
I'm surprised they say you can go with a 325 on a stock 11" rear wheel. I thought a 305 was the max?
I'm running a 325 on a 12" wheel.
If you go to Tire Rack, when you select a tire you can go to the "SPECS" tab and it will tell you the range of wheel widths that the tire will fit on. 325s will fit on 11" wheels. You can put 345s on a 12" wheel. I did it on my ZL1 1LE.