Wont give you any horsepower but I purchased a Short shifter from a forum member (
http://986forum.com/forums/diy-project-guides/54726-ball-bearing-short-shifter.html) bought a new shift knob and replacement boot all for around $225 and I absolutely love the difference. I think the aforementioned underdrive pulley and Top Speed delete pipes are probably better if your looking for performance gain but the shifter does increase the fun factor.