Well Sir, first off, in my basic opinion, if I were you, I'd call a Cummins service center or, cruise around on line, for Cummins Diesel pickups. I'm sure there's a dozen or so. There's forums for everything now days. If anyone's got info on how to "hot rod" Cummins engines, it would be them. One more place to call is, Gale Banks Engineering.
You see, Cummins allows for Banks Kits to be installed on their engines, unlike CAT, who does not and, there's no "kits" for it. I have a very good friend who has a '99 Fleetwood Discovery and it has the 275HP Cummins in it and, he installed a Banks kit several years ago. That coach is pretty hard to keep up with. He's real happy with that kit. Our coach is an '04 Itasca Horizon, 36GD with the C-7 330HP CAT and, I cannot keep up with him on grades. He pulls away quite easily.
Anyway, as for a straight pipe, I can't help you on that. I removed my big-a$$ muffler and installed an Aero 5050XL, Stainless Steel, straight through muffler and, I can tell you, it maid almost no difference at all except for the fact that it's about 30% louder now which, I hate. So, your choice Bud.
2004 ITASCA HORIZON 36GD, 2011 GMC Sierra 1500 4x4 Toad '18 Honda Africa Twin Adventure Sports DCT
Retired-29.5 yrs, SDFD, Ham - KI6OND
Me, Karla and the Sophie character, (mini Schnauzer)