I have a 88 GTU w/ a bonez replacement pipe with stock muffelers and it sounds just right. Ther pipe is a night and day difference from the stock cats. Now really how much difference can muffers make in actual performance numbers. This is my daily driver and I do not want it to sound like a weedeater. I am not on a budget but I cant see $600 plus making a world of difference for a cat-back system.

 

ON a NA the biggest change is the cat... usually you gain about 10 to 15 HP using a free flow cat. Since I don't believe in removing the cat entirly; I can't say how much more HP replaceing it with a test pipe as you have done helps.

The cat back is only about 5 to 10 HP on a NA, so figure $100 per horse.

Something you also may need to consider, is the series 4 motors like yours uses exhaust back pressure to activate the 6 ports, so you may actually loose HP going with a too free flowing exhaust, unless you make other changes to activate the ports.