Interesting. I'm on the fence about the Challenge system. The stock catback uses 3" piping all the way but it bends down to 2.5" in a few areas. The Callenge system also uses 3" piping but does not bend down to 2.5" at all. I'm not sure how much that could actually help.
I do LOVE the sound of the Challenge, just not sure I can justify spending an extra $1000 b/c I like the sound a little more.