i have always heard better to keep it stock as far as air filters go. Not looking for power gains, just looking at if it is worth it. stock filters arent cheap to begin with.
Stock filters flow just as good as a K&N or BMC street filter. but you can't clean them. once they are dirty you have to throw it away and buy another. I bought my K&N because I wanted toi buy it once, and clean it and put it back in. You have to buy the oil/cleaner which is 10 dollars and you get 4 or 5 uses out of it. I wouldn't go with a BMC race filter, it lets more dirt in. But that's my opinion.
Just for you information, a K&N filter will make you lose about 1/2 a horse because it doesn't flow quite as good as a stocker, but I'd rather be able to clean mine than worry about 1/2 horsepower.