I have a 2005 gt with long tube headers, off road x pipe, and flomaster axel back. I put a kenne bell on and it broke something in the mufflers. I got on the web site and it said not to use flowmaster axel back on supercharged cars. What would sound better the loudmouth or the power-flow.
the Loudmouth will be disgustingly loud with that exhaust setup. If you want to test how it will sound, take off your mufflers and run around with that setup first.
Here's a clip of what mine sounded like (shorty headers, o/r h and no mufflers) and it was crazy loud. I put the mufflers back on because I couldn't really think with how loud it was.
I don't know how much long tubes and x-pipe would affect the sound of the car. If by off-road you mean the cats were removed, than that will affect the sound. I would go with the PowerFlo's because I am not sure how the parts you have affected your sound. I have the LoudMouths on my car and am not sure I would want it louder than it currently is.
My TBird didn't get loud with it's mid lengths headers and X-Pipe. I am running high-flo cats on it though.
That said, Blowmasters are not a flow through design muffler and that is why you don't want use them for a SC application. You blew out a baffle inside of it.
It took the guys putting mine in a few days. they said they didn't know there was so many little things you had to move to install. they said at times there eyes would get crossed looking at the book for every step.