I bored the whole thing out to 15mm. it works fine, so leaks or anything. I think youll be ok. As far a performance it will probably just give you better throttle response and will add to the performance of the bike all around, because your cramming more gas and air into the cylinder. having a smaller intake though only a few mm smaller does equal to a decent amount of volume, so it allows more air and gas.
Like i said on the other post, just clamp it, line the hole up vertically, and drill away. It goes quick.