Remember that if you fit wider tires on your base boxster, the extra weight may indeed make it feel even slower.
I put aftermarket 18 inch Carrera "lights" on my car (not any lighter than the factory 17 inch rims, btw, but HEAVIER) and they're 1-1.5 inches wider on the back (can't remember exactly)...
Looks great, but performance suffers:
http://www.iwantaporsche.net/pixpage.htm