Custom-made full 3" exhaust. 3" downpipe, 3" main pipe, hi-flow cat, and a high-flow muffler (i'm looking at the Dynomax Ultraflow). That'll net a sizeable chunk of HP and torque, and shouldn't cost more than $500.
I have a 3" elbow off the turbo, a 3" downpipe, a high flow cat from ebay, and a Dynomax Superturbo muffler dumped before the axle. All bends are mandrel. I would recommend a tail pipe - around 3000 rpms and whenever I'm coasting, I get a very loud resonation inside. I can deal with it, but if anyone is riding with me, it's a bit loud to talk over when around 3000. And of course, I'm right at 2900 rpm on the interstate. Anyway, you can see a a pic of my exhaust at http://www.binaryweb.com/88lx/new_exhaust.jpg