I'm kicking around the idea of getting the Extreme Billet Air Cleaner. I like the look but am curious about how much of a performance boost I'd get with it on an otherwise stock motor (wold need remapped of course)? The other posts I've seen about it focus on the looks.

 

If you just do an AC, usually you don't need to do anything. ECM will adjust in a short while. Pipes & AC,,,, yep ya need a new tune.
If the air can't get out freely it won't take more in. Not a lot of performance gain alone.