cheapest (and safest and BEST) way to make your STI go faster is to learn to drive it somewhere even remotely close to its full potential.
spend your $ on developing the "driver software" instead of wasting it on half-baked bolt-on hardware. Subaru already hot-rodded the STI like crazy--do you really think you can out-engineer the factory?
You have an absolutely awesome car already, straight out of the box. Now it's up to YOU to learn how to get the most out of it.
http://www.teamoneil.com
high performance driver ed days thru CMC or bmw club, etc
try some rallyX/autox with an experienced racer riding shotgun, then ride along with them to see how it's really done
etc etc
DG