What's different with Corsa mufflers is it doesnt have any sort of packing, while being a straight through muffler. It relies on sound cancelation. Here's a video of it reving, after 19 seconds cut out is open and you can tell how much the corsa quiets it some. Gotta love that turbine whistle with the corsa and cutout!

 

Hands down the best I found is the Flowmaster part# 14020100 Pro Series 4". It is the quietest 4" I found that flows with almost no restriction. Its a big boy and does the job. I can speak from experience as I spent about 1k on mufflers trying to find the most quiet 4" performance muffler. Of course I ordered through Summit, so loud ones were returned.
Sound Clip Turn It Up!

 

Got a new custom Cam Motion 220/220 112 turbo cam in with same 4" Corsa muffler. Shout out to Mike Frumusa @ New Era Performance for remote tuning! Enjoy!

Cold start @ 865-905 rpm

Hot idle @ 750 rpm

 

Damn that sounds good!

 

Which one did you get prt #wise 8004000 or 8004001 it's shorter on the second one for a ford ir does it not matter

 

Thanks BlackGMC, I was looking for more chop out of a turbo cam.

Here's a link to the Corsa muffler, I used the one for a duramax. Keep GM with GM.

 

I had a 4" magnaflow kit for a duramax on my VHO with the turbo and it was the quietest exhaust I had on the truck other than the stock one.

 

Is it just me or do all of these seems super loud