The Staintunes with theire crossover is a big improvement, both in sound and power.
 
I have the crossover and a set of TOR's on mine.  Wish I had gone with the staintune pipes also.  they also save lots of weight.
Then there are the Preds.  3 innto 1 and I understand they are the cat's meow as to power and torque.
Good luck.