As Tim mentioned, the single 3" will still perform, although something a little more would probably be optimal. If you do decide to go true dual, I think that dual 3" would probably be overkill, as dual 2.5's should more than support 500+ hp.

I'd like for Tom fastnblu to get off of his butt and put his Maggie on his 5.3, as he's running Dynatech LT's and !catted! y-pipe, into a 3" Corsa cat-back. The Dynatech's have 2.5" collectors and y-pipe like my SW's, so if his setup will work out well for him, something similar should also perform well for your boosted 5.3, as well as my planned NA 6.0.

I'm pretty sure that for my projected setup, 3" collectors into 2.5" pipes, to a 3.5" cat-back would probably be just right, but as I constantly mention, I can't even afford to build my LQ9 that I've had sitting for over 6 years, let alone re-doing the exhaust.

 

I second that Billy!!!

Hey Tom get off your rear and get goin!

 

thanks guys i think ill do a tru dual 2.5 with a x pipe and some race mufflers

 

Before you consider the possible need to "open up" your exhaust side, have you already opened up the intake side? Or is the intake side still stock?

 

I found this....
How To Calculate Muffler Size and Exhaust Pipe Diameter | Exhaust Videos

I have a 2.5" true dual setup into flowmaster 10 series race mufflers ran out in front of rear tire. That being said I picked up a noticeable gain and had to add fuel and tweak timing in my tune after I replaced the single 3" Dynomax Bullet dumped before the axle setup I had. I dynoed a tad more then 400hp at the crank n/a with the mods in my sig with the 3" dumped setup.... No after dyno with the dual exhaust setup just Seat of the pants and the need to tweak the tune.

BTW: I have the Ebay headers with 2.5" ORY

 

According to the chart listed there, my current exhaust will be considerably too small for my projected setup

 

According to the chart listed there, my current exhaust IS considerably too small for my setup