The only true upgrade to it, I've been told, is that you don't have to replace it. It can be cleaned and reused almost forever. Actually, I've even heard it said many times that the stock air filter has better air flow than BMC or K&N, and that there is no real horsepower to be gained by going aftermarket. There was ONE aftermarket filter that was said to give you a few additional ponies, but I can't remember the name, and you had to order it from over seas. I believe the filter was made by a company called "DNA", but I'm not certain of that.
There has been a great deal of debate about this. Many have said that they were unable to measure an appreciable gain on dynojet dynos. Factory pro was the only co. in the past that claim to have measured gains with the BMC race filter over stock. The street filter had no gains over the stock one.
I run a BMC race in my zx14. It flows better than stock, but that also means it lets tiny particles into the airbox in order to have more flow. You might gain one or two ponies with the BMC race (if you also remap), but you do trade a little extra wear and tear on your motor(like most mods).
The BMC street won't flow any better than stock. Only advantage over stock with it is that you can clean it rather than replace it.
I had a K&N filter in mine for a while, but I found that it clogged faster. (If you compare the number of pleats in the K&N to the number of pleats in the stocker, the stocker has WAY more, probably 3 times more.)
Stocker did not clog in 24,000 km until I replaced it, K&N clogged up and needed cleaning in about 7,000 km. 'Course, the K&N CAN be cleaned ... but I'd rather just not have to touch it for a longer time. Back to stock air filter for me!
I have a BMC but I am do most of my riding on the track. With these filters I think there is more maintenance required as in cleaning them more to stay in the top performance range. I forget what the interval is exactly. But I clean mine after every track weekend. I think with any race filter it is that way to get the best performance. As mentioned you do sacrifice filtering for flow.
I put the race filter in my 08 and it ran poorly* until I had it dyno tuned. Considering that the yosh slip-on had been there since day 1 (and those were the sum total of mods) then it clearly had some effect.
No complaints whatsoever since the tune.
*[detonation at lower RPMS and surging between 4-6k]
You'll feel the difference going from a stock filter to a BMC race.......sharper throttle response and slightly quicker revving. Its not a night and day diff, but every little bit helps And if you are doing a few mods it all adds up