Reducing the weight of your vehicle is one of the most cost effective ways of improving performance. Reducing weight can impact everything from acceleration to handling. Depending on your goals, there are many areas of the car that are filled with "unnecessary" parts. Recently, Pat went through the trouble of removing about 80% of our STI's sounds deadening material in the interior of the car. In total, we were able to remove about 40lbs, with still more to go. Please note that doing something like this could greatly increase the NVH in the cabin, so evaluate your goals before undertaking something like this.
*In addition to sound deadening, our track STI has also seen removal of other parts such as AC/Heater system, rear seat belt assemblies, etc.
More details and DIY instruction can be found here.