Heres a vid of my RB20, 2.5" down pipe, 3" test pipe and 3"Fujitsubo NGK cat back, like Carl said drone is a little bit annoying on highway, but underpasses sound good.
Also, 3" will be fine for you, unless you really plan a major build in the near future. In theory the Turbo provides enough backpressure to get good starvation in the exhaust valves, so everything after that is just restriction basically. Do run a cat, like mentioned before a hi-flow cats flow well and you should use one. The I-6 sound is different too so a lot of exhaust setups that sound decent or average on a I-4 will sound way better by comparison. My SR now will get annoying a lot quicker Ive found, its a 3" down-test-RS*R
http://video.google.com/videop...37448