You hear all kinds of answers from bmc street, race to k/n race and street to just keep it factory... I have a race bmc on my g1 now but had always used factory before. As I recall Brock Davidson tested them out and found the factory breathes consistently better than the aftermarket versions. Unless thats been updated of course. Im going back to factory myself
Majority of guys I see run BMC whether its the race or street version. Most of what I read said K&N sucks and your better off with the stock 1. Theres also another brand that's out there that got good reviews PiperCross. Some GP race teams are running these
I think the only real HUGE benefit to aftermarket filters is the cost.Buy them once,they last forever. You of course have to maintain them. Stock filters are better if you would rather ride than clean. When they get dirty,just throw 'em out. My riding climate is moist and has very little dust,so I can get lots of miles out of a stock filter.
Stock in my Gen I... K&N in my Gen II. (thats the way it came)