When you changed from stock tire sizes to the 315's and corresponding fronts, did you notice any handling differences? I know the tires will be heavier. Any rubbing? Spacers? Lowered car problems?

 

315 carries the weight of the car much better, big thumbs up. Follow suit with 235 or 245 fronts or you'll be happy with balance.

No rubbing w 315, even at very low.
I ran my 996 with 315 on GT2 wheels, same offset but wider.
I used 315 with no spacers, and with 7mm spacers, both were fine.

 

same here. No rubbing with 12" GT2 wheels, 315s and 7mm spacers (fenders were rolled )

 

I ran 315/30 and 235/40 18in Toyo R888s at the TX Mile. They are mounted on HRE wheels with standard offset. No rubbing or any other problems.

 

you have to watch with the 12 inch wheels. if your suspension is in any way "a little tired" and you get some extra movement (probable track driving) the right side rear tire will rub through the wheel well liner and the boost hose.
for you guys who have not already done so check for wear marks on the right rear wheel well liner (been there done that!)