If they fit up front on the 9.5's they'll definitely fit on 8.5's so that's good to hear. You probably know this already but make sure you use the 8.5's up front if you're running Stock, the offset difference is fine but the width isn't.
I did some measuring now that I have the 295's in my hands and they're only about 5/8" wider than the 275 Contis I have on the rear of the car, and there's more than 1.75" of extra clearance back there. Looking good for the 315's. My measurements tell me ET29 is going to be just about perfect in front but the rear might need more like ET27 or so, which would put me in an interesting custom spacer predicament with the tire rack ET35 9.5's. Too bad those BBS Motorsport rear wheels from the GT4 are 10's as they would be perfect with a 15mm spacer.