[DLIT]

With seat time as a given...

Tires next. Once you have good tires, you have a solid base to work with everything else.

Suspension. Try getting it tuned to your weight first. You might be surprised.

Brake pads/lines. It's amazing how good adding good pads and lines helps. No more brake fade.

Exhaust/power commander. Have your bike dyno-tuned once you get the full system and power commander combo.

Gearing. This one last because it takes time to learn your bike. Gearing gives you the advantage of quicker acceleration, but you go through gears faster and less top end. It all depends on what tracks you ride. If they have short straights, gearing should help you out. Long straights, you'll get passed by all the guys you may have passed before the beginning of the straight. I'm still content with stock gearing on my bike.