Another thing about the stock h-pipe with 4 cats is moving from it to a high flow cat mid pipe will make things louder.
I had a Bassani cat x-pipe for shorty's when I had self ported E7's and just a few mods and it was louder.
After the current combo and the Mac cat h-pipe I could not believe how much louder things got, although this combo makes a bit more power.
Later
Grady