[Lucky]

Correct, 65mm offset will not fit the rear. At least not without a 20mm spacer.

I recently put Cayman S replicas on my car 18x8 front, 18x10 rear. Front offsets are 50mm, rears are 47mm. The rears fit, but clearance to the strut is tight -- probably 5mm at most. I haven't experienced any rubbing, but I'm running 265/35... I think 275s would rub. If you can find 10" rears with a 43mm offset, you would have enough room for wider tires.

Good luck.