it depends on what kind of spacers and the size. IIRC, Porsche recommends 7mm max in front, 14 or 17mm max in rear. Under 17mm, you can easily go w/ the 'sandwich' style which attach to the rotor hat w/ just one philips head screw each. Then all you need are longer wheel bolts (45mm + whatever size spacer).
sandwich style:
If you go bigger, which you might need w/ 10" in rear depending on offset, you will need the bolt on style. These have either a second set of wheel bolts or have wheel studs (they'll give you the lug nuts or extra wheel bolts). You use your orig wheel bolts to hold the spacer to the hub and then mount the wheels to the spacer.
bolt on style:
For track purposes, I'm not sure what the ruling is if you can use the sandwich style or not. If not, they remove easily. I suspect you are fine w/ the bolt on style at the track though. Since you don't want to run w/ them at the track, you need to remove them and carry an extra set of wheel bolts w/ you (unless you get the bolt on kind).
Regardless, get hub centric spacers - period. Also, I recommend getting the same size spacers front & back. This will help eliminate some of the Boxster's understeer plus it makes it easy as all the wheel bolts will be the same length. Personally, I put 15mm all around and I know some have gone upto 17mm w/o any issues. 5mm which Porsche uses occasionally from the factory is not worth it IMHO. 17mm is only 2/3 an inch so it really depends on a) your needs and b) the look you want to achieve.
Good luck
