feeding more air to the airbox won't do much if the exhaust side is still corked up . everything needs to happen equally , ie. more air in needs more air out and richer carb settings to all work in conjunction .
and on some machines the amount of air flow coming from the air box is already good enuff , because the cylinder head is what will determine the absolute maximum of air that can flow thru it .
to put it simply if you have a passage way in your head of a 1/2 inch diameter and you feed it air with a 1 inch diameter hole you are gaining nothing because the head is the restriction , understand?