Find a Box with a blown motor, buy it cheap, and drop in a 3.8L X51 crate motor. Now you can kiss all the 911's goodbye on the straights and devour them in the turns. This of course is assuming you have the driving skill needed to hang onto it. It's gonna cost a pretty penny.
Most of the other bolt on aftermarket HP mods for this car are pretty limited in terms of real world performance gains. They are extremely effective at emptying your bank account though. All of the top times for Boxsters in SoCal are still being laid with stock motors.