I just put on some BF Goodrich g-force sports on my rears and there awesome so far. My Kumho's were bald and i was in dire need of some tires, small amounts of throttle they would spin. G-force's hook up very well even on hard takeoffs. Only had the tires on for two days but so far i'm happy with them.
275's really too wide for an 8" rim. I know lots of people here run wider-than-suggested tires, but I'd still suggest going down to a 245 (stock width tire for cars equipped with 17x8s) or maybe a 255 if the tire runs narrow. I have 255/40 RA1s on my stock Cobra rims and they feel sloppy. If I stick to running the Cobra as my autocross car, I'm moving those tires to my 17x9s, and putting either 225 or 235s on my 17x8s.
I had 275/40/17 Nitto 555Rs on my stock rims at 15psi for about a month and a half without any problems. Id recomend spending a little extra for a better tire, though I didn't have any problems with the khumos that came with my car.