There are many things you can do to improve performance-
First on the list is to gain driving skill. This is done by:
Driving schools- several clubs have schools that are not limited to a particular type of car. BMW Car club of America has clubs all over the country that sponsor schools all the time. You can contact your nearest dealership for BMW and ask about BMWCCA. There are also many independent driving schools at all of the major tracks across the country.
With more driving skill you can take any stock car and run rings around any competition under any circumstance. It will take some time to improve but you will be much better after taking instruction.
2nd on the list is-
Get rid of excess weight. The lighter your car the better so don't add a ton of audio and video upgrades if you are interested in performance.
Subwoofers, amps, sound deadening material all weighs alot. Don't carry rocks in your back seat or trunk area. Keep it clean.
3rd on the list is-
Consider light weight rims and performance tires. This can make a big difference without changing from your stock suspension. Your wheels and tires are part of the total suspension system. The stock rims are OK but can be lightened by going with an aftermarket 14" or 15" rim. If you have lots of $$$ then a light 16" rim is also possible and looks nice but think lightweight for best performance. You can accelerate faster and brake faster with lighter wheels. Keep your stock rims for snow tires if you live in a snowy area or sell them or keep them for autocross in stock classes. Sticky performance tires will do alot to improve handling and allow you to go faster through corners since traction is improved. They may not last as long but that is the trade off.
4th on the list is-
Upgrades to your FIT. Stuff like a cold air intake (for the AT FIT this is coming- gotta wait a bit longer), Headers, Cat back exhaust, etc.
For the AT FIT the Cat back exhaust is a nice start. Watch out for the noise level- they do vary. You get a little more power and alot more sound. Drop in air filter (by Spoon and others) is also good.
Personally I like-
Driving school
Lightweight wheels/sticky tires- talk to
Jim@tirerack.com
If you have time go to autocross to watch and get inspired. Watch for those that drive stock cars with stock wheels and beat everyone.
It can be done and it's all about skill.
Last weekend we had autocross with 97 drivers and the fastest time of the day was posted by a stock Honda S2000. Great car, even better driving. He doesn't always do that well but that day we beat everyone.