Well, I wouldn't go with a "turbo" muffler or a flowmaster, they robbed a significant ammount of power when I tried them. You want a straight-through design. A cherry bomb will do fine if you have a turbodiesel, mine just didn't sound good because mine was a N/A. The longer the muffler, the quieter it will be, also a turn out helps resonation to a minimum.

BTW: I do have a flex pipe connecting the downpipe to the system. There is just enough space to keep it from hitting the starter. However, a longer positive battery cable is needed.

Just because the muffler has "turbo" in the name does not mean its made for turbo engines. Flowlackers aren't any better.