I built this a long time ago for my 350Z:
The two mufflers in the rear were very large straight flow through design, and for that reason along with the turbo, the exhaust was as quiet as stock. This is with aftermarket equal length headers, and of course no catalytic converters.
I have since built this:
3.5" piping from where the OEM Y-pipe would end, splitting in to dual 3". This is all aluminium, and weighs in at 12lbs. It is of course louder, but not as loud as one would think.
Also did a single 3.5", which is a bit more rowdy
it came in at about 8lbs :
Sound clip of the single 3.5" splitting in to dual 3.0"
http://vid137.photobucket.com/albums...psueffzhrq.mp4
Sound clip while on the lift single 3.5":
http://vid137.photobucket.com/albums...pscleukp0r.mp4