FYI, if you have had issues with getting a good balance on your wheels with your Boxster and it's still not right, find a tire shop offers road force balancing. I just had this done on my Boxster and the difference is amazing, my car drives absolutely vibration-free to 100+ mph.
This is similar to the machine my shop used:
http://www.hunter.com/pub/product/balancer/4159T/index.htm
This balancer is able to apply as much as 1250 lb of force to the tire to fully seat the bead after mounting, and to simulate road conditions. The system is able to identify the exact position the tire needs to be rotated to on the wheel to minimize imbalance, so that a minimum amount of weight is needed to bring it to perfect balance. It also measures tire and wheel runout and helps you ID problems with flat-spotting or other damage.
My shop charged $60 to do all four wheels, it took about an hour. As I expected, my left rear tire had the worst imbalance. Now, it's perfect. I did this on my M3, too, with similar results.