It took me awhile to do this. The Greddy is loud and it is a little embarrasing when I drive though my apartment complex late at night because it is pretty loud. It's Not unbearable and you can still hear your radio fine. It feels faster on the freeway. Punch it in 5th and it pulls quicker. Low RPM pull though feels the same as before.
The STS is my favorite. It glides perfectly into gears like before and the throw was set at the shortest throw level. The only problem is that it feels stiff and needs more effort to get it in gear.
As far as dyno numbers, I don't go to the track. I go off butt dyno. The exhaust might not make too much power, but I am happy with it. $500 to make me feel happy is better than paper numbers that I don't really feel(Y pipe).
I have a 2000 GXE 5spd w/ Y pipe, SI, 18 inch chrome wheels

 

I also have a 2k GXE 5spd sunlit-sand. Whats your car's fuel consumption? My mods are Pilot fogs, headunit, and bass box..and hubcaps (no-cost option I'd love to get rid of)