Yes I know these questions have been beaten to death, but it seems I can't find a solid answer to my questions. I've been searching the forums for a few days now. I have a 1999 Sierra 2500HD with a 6.0 in it. The exhaust has a little crimp/bend in it from backing up into a cement barrier. I am planning on getting it fixed, and I thought now might be the time to upgrade from the stock exhaust to something better since I'll be under there anyway.

Right now the stock setup has the dual pipes going into a single inlet on a muffler with a single out going out behind the rear tire. Will replacing just the muffle, with something like a flowmaster 40 or 44, and the pipe back give me any performance boost or mpg increase? Is there any real puprose to a true dual exhaust if I were to go that way, or a single in/dual out muffler?

Also, i was looking into a Volant CAI, and read someones post on here that it won't inrease mpg unless i get the ecu reprogrammed. Is that true? My plan for now was to get the exhuast upgraded and Volant CAI installed. Will that be any benefit or do I have to get it reprogrammed as well. Or would this setup without the tuning lower my mpg?

FYI - Right now I'm averaging 15 mpg and it mainly stays on the highway 90% of the time to and from work.

Thanks ahead for any input.