Matt that's a tough call the 275/35R18 A6 that I run is a bit more then 1/2 an inch taller in OD and that's about all your gonna get. The 315/30R18 (which you could run pending fender) would be the same OD as the 275 (25.6). Both Chris and i are getting a set of them in this weekend. his are going on rims right away cause he needs them for next weekend, but mine don't need to be on rims till after the break in June. if you want to mount up all 4 and see if it makes a difference feel free....no using them for the autocross though, i gotta save those babies. but at least you'd see if that extra bit will help.
That's the hoosier option.
the kumho option is the 315/35R18 which you should be able to get on a 9.5 alright. they are a 26.5 OD is very close to your stock OD.
KEEP NATIONALS FROM CONCRETE! CAUSE I NEVER RUN ON IT!
2007 SCCA National DS Runner-up
Badlands SCCA
2007 Nationals Runs