I have not run with a different exhaust. It sounds good with the Ultraflows, although I'm not sure I'd want to take a long trip with it. It has a howl that sounds very exotic.
I was searching for information about the optimum crossover area so I could fab my own x pipe and ran across the attached article.
Its not obvious in the picture, but the tubes run through the indents in the trans crossmember. The tubes are 3".

EDIT: I am running 2.5" tailpipes from TTI, not downturns.