I changed the brakes on my '99 to 2001 S "big reds" with cross drilled and slotted rotors and noticed the difference immediately. On the track is where I really noticed it as just a tap on the brakes slowed it down instead of the heavy pushing I had to do with the stock brakes.
The rotors, calipers and discs will fit just fine with 17" wheels and, as has been mentioned, you will need to change the rear axle housings to the S housings. Your stock axles will fit fine but the diameter is smaller than S axles so you will need to make a bracket that holds the sensor for the anti lock and air bags deeper into the housing so it can read the rotation of the axle.
You won't notice much difference in AX but on the street and track they can really make a difference and they look great too. Just my opinion but they are one of the best upgrades I did for my Boxster.
