I have put in a K & N Filter and drilled the holes in the airbox, it works fine, but at the same time I drilled out the exhaust and the bike performance has improved on the acceleration and the hunting at lower speeds its a lot smoother and not so jerky on the throttle, only slightly negative thing I would say is that when you deaccelarate you will get backfires, going down a hill for example. I checked with the riders in the USA and they get the same thing, you can get rid of it but need to buy appox £200 of performance controller, me I put up with the banging.