[thstone]

Don't forget that changing the tire stagger (the difference between the width of the front tires vs the rear tires) will affect handling.

The 17" stock tire widths (205F/255R = 50mm difference) were selected to ensure that the car would understeer at the limit. Adding more stagger (205F/285R) will typically result in even more understeer (I would expect a 205F/285R car to handle like pig, i.e., terminal understeer).

Most Boxster owners reduce stock stagger to 30mm by running 225F/255R. This reduces understeer and moves the car towards more balanced handling. Many track cars run 235F/255R (20mm stagger) to further reduce understeer. And yes, some track cars run a so-called "square" setup of 255F/255R (zero stagger) to dial out most if not all of the tire width induced understeer (this setup also typically requires some other changes (alignment and sway bar settings) to keep the car from too much oversteer).

So remember, when going wider in the rear to also consider going correspondingly wider in the front to maintain whatever stagger (and handling characteristics) you prefer. This will keep the car's overall handling characteristics relatively constant.