Different front and back wheel sizes.
 

Is there any benefit/problem with having 16" wheels on front and 17" on rear. I know the've done it from the factory on the NSX and some other cars but would like to know how it would effect 911's.

Specifics: front 245/45/16 -- rears 275/40/17 on 930

Actually, i have it vice versa, well the tires anyways. I have 16" 225/50 at back and 16" 205/55 at front.

Shouldn't be a problem other than looking strange. 911's did it by the way with the 935.

Your front dimentions are 9.64w x 24.68h.
Rear is 10.82w x 25.66h.

This puts you an inch taller in the rear than the front. You may wish to lower the rear in order to give the car the desired rake.

The design of our "a$s draggers" means that the air gap between the tire and fender will be much more pronounced on the front than rear with this setup.