I have the +15mm kersher fenders. I have only fit a square 255 setup though. My problem isn't rubbing on the fender. The problem is body roll causing rubbing on the bumper when cornering. I am not sure whether or not it would be a problem for someone else starting off with less body roll (it was bad with 255s and stock suspension).
Specs:
Alignment: -1.5* camber (m3 arms and alignment pins pulled). 0 front toe. 7.6* caster. More camber coming now that i havr ycw coilovers installed, but rubbing the top of the fender isn't even an issue.
Wheels: square kosei k8r et45 17x9 with 255/40/17. et38 front (7mm spacer). The strut body of the ycw is thicker that stock so I'll have to move to 10mm spacers (et35) this spring.
Here is a pic showing where the wheel rubbed... it's not noticeable at all but you can see where sone of the bumper is worn away if you look for it. This happened at lime rock all in one afternoon due to body roll on rock suspension:
I'd think a 265/35 on a 17x9 might work well if I wanted to try it. I think you need to move to a 9.5" wheel to support 275s correctly.
I think 265/275 on 17x9.5 might fight square. It would be very agressive fitment up front though. Here is the minimum you'd need to even try it:
Front - et30 or lower and make sure you have adjustable camber plates.
Rear - et52 - et58 or higher (careful not to go too high with high camber or you'll rub the fuel filler neck)
I have my doubts that 275 would fit right up front though because that puts you at +20mm more poke whereas the Kerscher fenders are only +15mm. The fenders also taper back to stock sides skirts and bumper so it's less than a 15mm flare in places...
Basically, 255s fit stock as a maximum. Then +15mm allows for 265s and the extra 3mm poke I need to clear the ycw strut body (268mm total).
Also consider that 18's and 265/275 tires will have a larger diameter than stock further pushing you toward rubbing. You'll need a lower profile tire like a 30 series. Even 18/265/35 will result in a significant increase in tire height. This is why I suggested 17's. Problem with 17's is that you cant find any street legal tires that come in 275/35/17. The ones they do make are crazy expensive. Pricing on the tires alone alone makes 255/40/17 a way more ideal fitment!