With the stock H pipe you won't find a better sound then doing this. Even with a catted pipe it dosn't sound bad at all. If you do this with an O/R pipe you better look for something else.
I haven't on a mustang but I did on a 95 Lightning and it sounded pretty good. But I would guess it would sound like some SLP LM1s cause they are nothing but a hollow can.
i orginally ordered a MAC cat back for my 03 GT and when it got to my house, tore off the exhaust from the cat-back. then i opened the package, wrong system... i drove around for about 6 weeks with nothing but the stock h-pipe and it sounded incredible, i was surprised it didn't sound dirty. after fighting with the company for those six weeks i completly canceled my MAC order and got the SLP loudmouth 1 system from a different company. similar sound to the h-pipe only set up but it is much, muuuch louder than the LM1. very happy with the LM1 but i miss my car shaking at a dead idle like it did w/ no muffler