We had good scuccess running 245 tires on 8" rims. It does change the gearing, and ride height abit, but still seems to be a net gain. This info is from the days when the enduro classifications gave us the options to run under GTS rules in the same class as Spec (before 944 Spec was moved to E3).

If you want to be ambitous, and spendy, 245 A6/A7's's will be the fastest during the short qualifying races. A fresh set *might* be the ticket if it's not too hot (which it probably will be). Hoosier R7's will likely be the fastest for the longer champ race, though a set of 245 Hankook Z214's will be close for less cost. We haven't found the BFG R1 to be much faster, and it's harder to drive. The R1S might be better, but could overheat in the long race as well.;