Yeah, they're +17. You should be able to fit 275's in the rear just fine, you'll have to roll the fender lip up though just to keep the tire from catching. No flare needed! Depending on the sidewall stiffness of the tire you run, you'll get enough flexing to rub on the muffler heat shields, I'm not sure if that's okay with you or not.
My inspiration is Blue TII on the other forum, I wouldn't have attempted to run the 255's if I didn't see his thread.
Here it is on the other forum:
http://www.rx7club.com/showthread.php?t=772831
The only reason I pulled the fenders is because of the +17 offset on the wheels, I only did a very slight pull on the old wheels. Damn I still miss that car.