Smoothing out the casting will improve the airflow and give you a few hp at top end, but should also give you a slightly better / quicker throttle response.
I did mine last week and there was a differance in throttle response.
Don't go too mad otherwise it will end up a bit jerky at lower revs.
There's a thread on CGTI about breathing mods. They could find no differance on back to back rolling road tests between performance and standard new paper air filters, but this would change slightly with an older paper filter. There was an improvement with a smoothed and modified air box. i think they are going to look at the smooted throttle bodies next.
Paul