If you buy a K&N (like I did),you'll see the filter media is smaller than the stock unit. I would assume they flow the same or only marginally better than stock. The only advantage is that it can be cleaned and re-oiled and you'll save the expense of air filter changes. Oh,yeah.....you get a cool K&N sticker to put on your bike!!!!!hahahaha
Id get the air filter and a PCIII. I cant claim any HP gains, but I know my bike pulls harder since me mods, and I have to think my BMC air filter, LV pipes, and PCIII helped... git'er dun!
BMC filter has more flow area than a K&N. Plus BMC makes a "race" filter also. (and BMC stickers are cooler than K&N stickers. K&N stickers are so................yesteryear.)