Get 18" winters
Anyway, for the street, your tires are the biggest inhibitors for better stopping, not the brakes and certainly not the rotors. On the track where consistently repeatable stopping power matters, things like more swept area and mass for cooling matter.
If you just want 2 piece rotors b/c they look cool (nothing wrong with that if you have the money), go with Performance Friction. They only cost $730 or so for the pair unless the aluminum hat of the girodisc is worth $1500 more. Plus, replacement rings for the Performance Friction are only $200 each. The girodisc looks really cool and I'm sure it is high quality, but I can't justify the price.
http://www.ogracing.com/catalog/2-Car/28-Brakes/240-Performance-Friction-Brake-Rotors/242-Porsche/item-1467-PERFORMANCE-FRICTION-DIRECT-DRIVE-ROTORS-PORSCHE-996-997-BOXSTER
Threaded Mode
