I don't think the x-pipe will do anything for a rear-exit exhaust, although I know somebody around here has one cause I remember the pics.
Its done on side-pipe cars so you can hear all 10 cyllanders. If you've ever been on track and heard a side-pipe car go by it sounds very odd (even from the driver's seat). The SRT-10 added the x-pipe to sound better.
I think my X-Pipe made a big difference, helped the sound a lot and cut down on deceleration "popping". I was running straights from the cats back and added the X-Pipe, I like it much better. No SOTP gain, no gain at all in power just much better exhaust tone and smoother decel. :twocents: