You're gonna have to do some researching on custom wheels, as the backspacing will be different than stock. If you use a larger, wider wheel, chances are you'll need spacers. These links might help you.
944 Wheels FAQ
14.0 Performance Modifications
I personally love the cookie cutters, but couldnt stand the wheel gap in the rear. So, what I did was swap out my studs for longer ones, with the factory spacer and a second hubcentric spacer. Open ended lug nuts too.
Ive read not to go any larger than 17" wheels on a N/A 944 because it'll affect peformance.