I have flows and a prochamber on my 01 gt, and it sounds ok. I didnt like the way it sounded before the prochamber though. I personally prefer the sound of MAC mufflers. You can go to
www.mustangexhaust.com and listen to several different combo's.
Here is the slp with stock manifolds and stock h-pipe on a 2003 gt, launching.
Here is another clip of slp on an 03 with the same setup, I guess the difference is one is launching the other isnt.
Here is a clip from a from a 00 gt with 2 chamber flows, stock h-pipe, and stock manifolds.
Just for good measures,
here is a clip of a 00 gt, prochamber, stock manifolds, and MAC flowpaths.
Hope this helps.