Just came back from the NY Auto Show and they had a great setup for the new Mustang, with cutouts of the engine and a sideways car so you can see the underside. Took a couple pics of the exhaust. You can see how right after the second o2 sensor the pipe narrows down considerably, I would say less than 2" and it stays that way for a good 10-12 inches to right before the h-pipe where it goes back to 2.5". That should be a good mod power wise. Also, in the last pic, where the pipe bends over the axle there's a nasty kink that could also slow down exhaust velocity.
And for good measure, the hottest twins at the show courtesy of Fiat:
And for good measure, the hottest twins at the show courtesy of Fiat: