After a recent visit to have my bike Dyno'd i was told that there's not too much difference between a decent stock paper air filter compared to a K&N on the 600 Fazer's, although the K&N certainly feels to me like it breathes easier, and economically makes more sense as it'll be the last filter you buy because they are washable with a million mile guarantee!
The K&N filter, along with the Yamaha filter, will also fit a damn site better than any cheaper aftermarket one's you'll find on Ebay. The Hi-Flo one i had in before had about 10mm play in it, whereas the K&N was an exact airtight fit.
I have an SP Engineering Moto GP end can, that has British Standard markings on, and as long as the baffle remains in place is perfectly road legal. (not my bike in video but same end can)