I was looking at my 300EX and noticed that on the air box the intake goes through the frame. So I tried riding without the air box lid to see if it made any difference. I couldn't see a real big difference, so I was wondering if it is even worth drilling holes in the lid for better flow?

 

I took the lid off mine once too and I could tell a little difference. If your going to modify the airbox it would probably just be best to take the lid off completely.