I used to want to do a dual exhaust conversion too, when I had my integra, but that car was all about performance and so taking a hit on hp or torque wasn't gonna happen. basically you're on the right track. if you have both pipes at 2.25" (for na performace) then that's way too much room for the air, however if the pipes are smaller then they are supposed to be say 1.75" (making a number up so obviously this means nothing) then you could get the effect of having 1 2.25" pipe. You gotta do your math to figure out what the magic number will be. but yes you can do the dual exhaust look and not lose hp. Another alternative I thought about way back when was to just hang a dummy pipe, I never thought about it all the way through, so there may be problems involved with that. (ie maybe the sound would be really wierd only coming out only one pipe, and one will get dirty because of fumes, but the other won't, I don't know just quickly thought up ones) Good luck