I run the TTI stepped headers on a built 340 and just installed a X-Pipe with flow Masters and it made a nice difference. The car still has a nice growl and with the X-Pipes and the Flow Masters, I now run the same times in the 1/4 mile that used to run with open headers. It's tough to put a HP number to it, but with my setup I would say the 10 to 15 HP gain is reasonable. If you do go with the X-Pipe setup bear in mind there are several ways of doing it. The pipes can literally cross, or you could also cut a section out of each pipe and have them joined for a few inches, but still maintain the separate pipes.