I just got some 18x10.5 wheels square delivered and plan to run 275/40 for now, with Nexen SUR4Gs.
Depending on how that feels at track days, I might go with 285/35 Hankook R-S4's next time around. Sadly, RS4s are sold out in 275/40.
I ideally wanted 295/35 but selection in that size is pretty bad