a 275/40/17 up front and a 295/35/17 on the rear will fit nicely. you could go with a 285 on the rear, but it will stretch a bit and theres no real advantage to running only 10mm wider in the rear. a 315/35/17 will fit on that rear wheel and there will be a lot of options in that size.
its actually easier to find quality tires in those width in 18s. Michelin has a ton more options that are N2 aproved(N2 is the Porsche quality approval rating).
