i like k & n air filters and think they are the best. i run them in all my vehicles along with mobil one synthetic oil. now that is going to spark some arguement........which i have been in before but to lazy too find the link on here (feel free it was from a couple of months ago).... i put my k&n on my 00 r6 and it runs just fine maybe even better but i dont think a person could really tell unless dynoed out somewhere. some people have problems with them.......probably the people that have all stock equipment. me on the other hand have full akra exhaust and the carb is jetted. it all comes down to if you want to try to experiment with your bike and make an investment of $50. if you like you'll never have to buy another air filter again becuz as you probably already know they are usable :boob
I think if we started asking who had put a K&N on a bike that didn't run better I think we'd find that they are mostly carbed bikes and weren't jetted either to take advantage of the increased airflow. I haven't seen any test of just straight airflow across the filter (stock vs K&N vs bmc) but that would be interesting. Also, on the FI bikes, I think they benefit from the newer filter elements since the stock ecu can adjust somewhat for the increased airflow, even more so with a PCIII. I have noticed that yamaha now sells race filters on their site for the 03+ bikes.....interesting side note as well, they have them pictured with V-Stax but not sure wether or not they come with the filter or not.....I myself have a BMC Race filter that I am going to be fitting next week with my slide stops, PCIII and V-Stax.
I am willing to bet that the K&N flows just as well or better than the stock filter. Like Pen pointed out, whether or not the engine can make the best of that flow is another question. Here is one thing you all may have forgotten to consider. If the hp gain is only in the range of a couple hp, and you don't need every last drop of power (street riding) the K&N may not be a good idea. The reason these other filters flow more air is because it has larger 'holes' if you will. It allows more air through, but at the same time allows more and bigger particles through that you may not want in your engine. For a car engine which is suited to more abuse it isnt a big deal, or a race engine that gets new rings and the valves lapped every season. But for a 15k rpm street engine, that isn't a risk I'm willing to take. To each his own, I'm not being pushy, just throwing out a point you may have forgotten.