I ran MBRP's axle back for a year or so. It looks good and fits good, if you can find a deal on one go for it (and IMO it would probably fit a lot better than the Pypes). It's been so long since I had the factory cats on my car I can barely remember, but I do recall it sounded good outside the car but was not bad at all inside the car. Pretty mellow for a straight through axle back until you get on it, then it barks!
I do have a video of my car from back when I had the factory cats with the MBRP's...just idle and revving, but I'll post it if you're interested (I'm at work, can't get the link right now, youtube is blocked)
'05 Mustang GT Premium
CMS Stage 1 cams, C&L Racer, Lito tune, Jeg's oil sep, CMDP, SR UDP's, JBA stainless LT's & catted H, Magnaflow resonators, MBRP axle back, BMR springs, Koni STR.T, BMR LCA's & backets, J&M upper & mount, J&M Al panhard, DIY panhard brace, Steeda Tri-Ax, smoked corners, Raxiom Vector tails, Brembos, S550 GT Premium wheels, 2010+ door panels