Les - It's all a question of balance. I think you'll see most rear and mid-engine exotics have bigger rear to front ratios than the Supra (front engine, rear drive). Most people racing Supras put the largest tires and wheels they can on the car because it has the horsepower and suspension to use them. You'll notice people still pretty much conform to the stock rear to front ratios as the stock setup - i.e.- you don't see people racing on 295s front and rear. 275s or 285s in front and 315s in the rear seem to provide a nicely balanced combination for the Mkiv.
Jeff
Jeff