There is not a lot for the Sti I would work on the ecu mapping first turbo back could be real harmful. Two words boost spike ooouuucchh!!!
Sti is not a wrx I you are looking for horse power you have it if you are looking for noise get a cat back and a up pipe! But do your homework ask
S-squared nate knows his stuff Cobb is coming out with stage packages for the Sti
cobbtuning.com/sti/