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.
Rubber side down! :cheers