Frank, As far as I know "18x8.5 Front and 18x10 Rear" is the only Boxster 18" size. They will fit all Boxster wheel wells.
I have had the 18x10 rear on the front with stock 265/35-18 tires without rubbing. The grip while turning was fantastic! Others have rubbing problems with 255s in the front. Noone has reported problems with 245 or less in the front, using a compatible rim width. Depending on the tire, a 245 tire may recommend 18x9 wheels - some 245s work on 18x8.5.
Unless you have an upgraded engine with massive torque, the 18x10s are wide enough to provide rear traction. In my experiment, running the front tire(225) on the rear provided dismal grip. Others have run up to 285 in the rear, and then one really should go to 18x11 for the sake of the tire. I think the 265/35-18" rear tire on 18x10 is the best.
There are folks who prefer the unsprung weight of 17" wheels. I prefer the crisp turn-in and wider tread of 18" wheels. 17" tires are less expensive.
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