I have a dynomax on my 81 turbo. Love it, nice deep sound but not as loud as the ratty sounding hondas the kids have.
I used a 3" pipe from the down pipe to the cat and a 2 1/2" back to the dynomax near the rear bumper.
Hard to say if anything really improved because the old muffler and tailpipe were full of holes and the car ran like sh*t when I got it.
I have a DynoMax SuperTurbo on my 74 260 Z with no catalic converter and a 3" exhaust pipe. It has a nice smooth sound and not that loud. (Well kinda) You may get different results from having a Cat converter.
Thinggggs are getting stiiicky! **** the soda. As I was saying, I use a Magna Flow 3" in 3" out straight through design muffler with basicall a 3" medrel bent cat back system with out the cat on my 240Z Turbo. It sounds great and the car pulls to the rev limiter easily. I have also heard good thing about the Sebring straght through with swirl design.