BMC race filter for me.
Never done or will do anything else
I had a K&N and then a BMC Race.
The BMC Race is almost identical, only it's less than 2/3 as thick as the K&N.
As soon as I originally put the K&N in, I could hear a difference through the gas tank.
As it obviously flows more air.
To me, that is proof enough that an aftermarket filter can make more power when properly tuned, as more air/fuel in= more power out.
Years back there was a gen1 rumor that the stock filter made more power...I have yet to see any proof.
To our dyno tuners(not gonna call anyone out), but, if you guys ever get the time, would you do a before/after dyno tune with a stock vs aftermarket air filter? Please.
I'm also just talking numbers, not filtration.
As the thinner and more pourus the filter material is, the more air gets through, and so does more dirt.
So stock would have to filter better.
Regardless, unless you want every paper hp you can get, the real benefit of aftermarket filters is being reusable.