I did this trick years ago to a Civic and the guy that told me about it claimed it gave him 5 more hp on the dyno.
I recently did a valve job on my Vibe and realized by looking at the throttle body diameter and comparing it to the snorkel going into the airbox that there was a major choke point. I went looking in my local boneyard for the lower half of the airbox and grabbed one from an 05 Corolla (tbw!?) for $7.
I plan on cutting out a few panels on the bottom and rear of the airbox bottom to reduce the restriction to the filter and hopefully make the engine breathe better.
The only effect on the Civic besides the desired hp gain was a neat little howl on acceleration, but that motor had a resonator on the intake tube before the airbox.
Before I do this, I was wondering if there was anyone out there that already tried this mod and how it came out?