The Boxster kit increases thermal stress on the stock rotors. The car will stop better initially, but repeated stops will actually exacerbate fade. (So I understand. I believe Bill will corroborate.) Edit. Ack, I see he did.
My use is virtually all street. However, we have several nice mountain roads nearby, and I've been able to overstress the brakes on long, twisty downhills, even with premium pads and fluid. So I'm interested in practical upgrades.