SSI heat exchangers and a bypass pipe is the first thing I would do. that will get you close to the ROW 204 HP rather than the 180 something for US versions. I am assuming you aren't in Ca. That will cost you about 1500 or so. From there additional HP gets more expensive. You about have to start on the cams and other internal upgrades. You can do a super charger kit, but I wouldn't really recommend that unless you rebuild the motor first, and then only if you are a real speed junkie since you'll need to upgrade the brakes, suspension, etc if you get upwards of 300 HP. The SC is light enough and well balanced to be a lot of fun like it is, minus the US emissions requirements of the day.
2003 BMW E46 M3...2001 Boxster S 3.6....94 968 Cab...80 911 Weissach Edition