I believe the only reason it is on there is to satisfy the external radiated noise people in Washington. It dilutes some of the intake velocity noise. Taking it off should only slightly increase the incomming air sound (ala CAI sound). No performance increases. Plus you can see more of the engine
I removed my resonator but I didn't plug it; I put a drainage type plug in place instead that I bought from Lowe's. It seems to work well, no check engine light. But we'll see what happens when I drive around a while.
I took mine off and plugged it with a #9 1/2 rubber stopper I picked up at Lowes. Been that way for over a year now. It doesn't help or hurt the performance of the car, it just looks ugly as hell and blocks the view of the passenger side valve cover. It was originally put on to reduce the transient drive-by noise of the AUTOMATIC cars for EPA certification, but instead of just putting it on the automatic cars, it made it's way onto ALL the cars.