From a 315mm wide tire to a 335mm tire (20mm), I dont think you will gain any significant amount of traction both sizes being the same tire manufacture and model. heck I went from a 285 to a 315 and didnt notice any difference other than the looks, my traction still sucked. Its all in the tire choice, probably one of the best tires for dry straight line traction was the mickey thompson drag radial (new design).
Or you could focus on the suspension to get maximum traction for the given tire. For example if your supra is lowerd a good amount (over 2") you will most likely have a good amount of negative camber, say -1.5 deg. With that much negative camber a 315mm wide tire wont help you all that much as upon launching your rear end will sag an inch or so which will give you around -1.9 to -2.1 deg of camber. You will only use up to 85% of the tread face. So if you go with a 335mm tire you most likely wont be using that much more tire to make a difference.
If you dial the rear camber a bit positive (-.75 to -1degree) you should see an improvement in straight line traction. Just my $.02
Lawrence