A few things...
1) The GT500 wheels are all 18x9.5, with a +45mm offset, only the tires are staggered (255/45-18 front, 285/40-18 rear, spare is a can of fix-a-flat...)
2) Grip has little to do with just pure "numerical width" as published by the manufacturer: My Hoosier 275/35-18 tires are actually WIDER than my old 285/40-10 Nitto 555s.
3) Compound has a MUCH larger impact on net tire grip than pure width. A sticky 275 will kick the snot out of a high-mile 295 or 315 tire, all other things being equal. The FR500S runs a 255 tire... (BFG R1)
4) In the quest for grip, I've come to the conclusion that it's the full combination, and not any specific part that makes or breaks the car's ability to grip. Yes, width helps, but so does compound, tread-block depth, damper and spring ability to keep the tire planted, and swaybar rate.
In the end, a 275 width tire, with a decent compound and tread design (not talking slicks or DOTs here) will be more than sufficient to allow the car to plant in a corner, assuming reasonable HP. From there, it's up to the dampers, springs, and bars to keep those 10-1/2" meats on the ground, and not flopping around. Having steamrollers (the elusive 725mm tire!) won't help if you're lifting a tire off the ground in the corner! Also, think about all the discussion about racing differentials, and how a failing diff can cause the car to oversteer. THAT can't be corrected with tire width!
