i don't know about yamaha or kawasaki... but suzuki and honda stock filters are just really good from the factory, the K&N when clean flows less, when its dirty it might flow same as stock or slightly better but it always allows more dirt through in independent tests... but like i said thats just honda and suzuki, i think kawasaki's actually take well to K&N's, and yamahas i have no idea... but its not a big deal at all, they don't ruin the bike or anything, but some people like me are anal over 1hp and 50 microns of dirt

but i spoke to a guy at K&N once who actually had me pretty sold on K&N's being good, had some really interesting flow charts and things like that, but those can be easily made to appear one way or another, but the same goes for the independent tests, i just trust they are a little less biased