15s will fit on the rear with non Cobra brakes and SHOULD fit on the front with the regular GT brakes. However, it will look effing stupid. The stock wheels at around 25 and a half inches are small enough, looks like a set of damn roller skates on the car. I run slightly oversized wheels at 26.3" up front and 26.6" out back on 18" staggered wheels. 235/45/18 and 275/40/18. Plenty of sidewall, comfy ride, fills up the wheel wells nicely and looks x1000 better.
Also the tire size has little to do with 1/4 time as to overall gearing. If you want to run taller tires just run a steeper rear gear to compensate. I could throw 4.30s in this thing and still have a good cruise on the highway and come flying the hell out of the hole even with a NA 2V and bigger rear tires. I just might do that with a .62 overdrive. Your overall goal with the 1/4 is to be hitting the traps at the top of 4th gear anyway so just calculate accordingly to whatever size you choose.