I had a similar problem with my modified SRT6 with stock exhaust. It turned out to be loss of traction and serious tire squeal when the right pedal was depressed to agressively.

Seriously, I did once put a u-shaped pipe just like the one pictured in a TVR once and the sound could not have been improved. I never considered a V vs. a U making a difference but I imagine there is some mathimatical reason it would.