I personally have found no good reason to drill bigger holes in the air filter housing, these little engine can get plenty of air through the holes that are already there in my experience, just a free flowing exhaust system and a correct jetting on the engine after it is broken in good makes a world of difference all my bikes cruise @ 30+MPH real nice with smaller sprockets, a free flowing exhaust and a good tune on the NT carb.
I did shave some material off of the heads to boost compression a bit, but mainly I just have them tuned pretty good to where they aren't lugging down and 4 stroking from being to rich, with a good flowing exhaust.
these little engine don't really have to be all that fancied up to perform well if they are just set up right in the tuning.
Like I said, I don't see that you would make any gains by drilling the air filter housing, all that will do is let water in if you ride in the rain or run through a bunch of water.
Peace, map
If i were to drill a couple holes in my air filter how big should i make them???