What do most do when installing a 4" system that has a converter, such as my 2003 Duramax? I want to stay legal and not void the warranty. Is the usual procedure to say heck with it and just have a catback, or are the pipes fore and aft of the converter necked down? How much effectivesness is lost with a catback? Thanks.

if you are concerned about legalities then just get a cat back system and be done with it. the stock pipes are 3.5" but you can still bolt up a 4" cat back at the flange with no problem. also keep in mind if you have emissions equipment on your truck..i.e. egr along with the cat.. then you will need to get an EGR blocker plate and a finger stick to prevent any trouble codes and performance issues by removing the kitty.

you will gain lower EGT's by increasing the pipe size and mainly by removing the kitty, but a cat back will still net you some lower temps and ok gains.

if you replace the kitty make sure you keep it in case you have to go to the dealer for anything related.