Auto-X tires and rim widths

[El_Tortuga]

2014 GT c/w Track Package, and I'm wanting to play in STU autocross regularly and a few track days a year for fun. I'm doing the ER event this September and will probably just run the stock summer Pirellis for a baseline.

Car has been lower 1/1.375" with Eibach Pros and Whiteline PHB. As a daily driver, I'm reluctant to go much further down. Only other planned mods are Konis and adjustable camber plates.

Dropping down to 18s would seem to be the thing to do. Good reviews on BFG rivals and the closest they have to the STU max of 285 is 275. I doubt that I choose to go play in ESP, but might like to have a wider tire width option if practical.

Any suggestions on rim width and offset?