I'll stick to your original question and say the best performance mod you could purchase for your car for least amount of $$$, B&M short shift kit. best low$$ investment on my 986. Makes the car a bit more enjoyable to drive, and your shifting downtime will be a snick quicker. After that I would say the guys suggesting driving lessons are the correct. As for the wieght issue, For me I would leave the cars weight alone and try to lose 20-30lbs myself!
Steve