[Perfectlap]

Well personally I would avoid spacers, but that's ony because I intend to track the car and attend some driving schools. Porsche naturally doesn't sign off on it.
But Spacers would be needed if you trying to retrofit a 911 rear wheel to the Boxster. That's a 10" wheel.
Better to stick to the Boxster widths. Check out Wheelenhacement.com they have a very good menu of wheels that will fit and they will answer any questions via email regarding spacers and lugs.
Just a quick note, if going bigger (18") try to get a light weight wheel. You'll get the looks without any big change in ride quality.