As most of you know, I'm running Simmons B45's 16x8 fronts and 16x9 rears. When I was doing trackwork I had a set of Simmons FR17's with track rubber. The fronts were 17x8 (4+1/2" or 115mm backspace) with 235/45x17 tyres, the rears were 17x9 (5+1/8" or 130mm backspace) with 255/40x17 tyres.
Exactly what these relate to in terms of an offset number I do not know. In any case the fronts were perfect on a reasonably lowered car. The rears looked like bicycle wheels!! I can fully understand how people can get really wide large diameter tyres on their coupes with a low profile and hardly any sidewall flex.
See pics below:
The inner sidewall is fairly close to the rear springs and tubs but still enough room to prevent scraping. I had no problems whatsoever at Winton with this setup.
The clearance between the outer sidewall and the inner edge of the guard was huge, I could pretty much get my fist in there. The measurement was (from memory) 60 to 65mm.
Going by this, at this offset it would be possible to run 17x11 with 295 or perhaps 305 rubber and probably not have to lip the guards.
Anyway, hope this info is of help to anyone out there.
Cheers.