The flow improvement of the K&N (if any) over OEM is insignificant on modern mostly stock engines. The filtering efficacy of the K&N is a bit less. If you concur with AnthonyS's veiws and testing methods (I sure do 100%). The K&N’s I have used on modern mostly stock bikes made no noticeable, seat of the pants, change at all. Certainly at lower RPM’s and lower air flows there will be no change in flow or engine characteristics.
“I choose to conduct this test because of the conflicting information I see in advertisements and have read on the internet. Everyone claims that their filter flows the best, and removes the most dirt. If you think about it, flow and filtration ability are actually linked. A solid piece of metal would prevent any particles from entering the engine, but it wouldn't flow any air at all. On the other hand, the screen used on a window screen will flow well, but won't filter well at all. So if you think about it, the best flowing and best filtering is really contradictory in claim. I wanted to find out which filter really does filter the best, and which one really does flow the best. I have used almost every brand and type of air filter over the years including K&N. I had not used a foam filter until conducting this test. I have used every manner of off the shelf fibrous or paper filter.”
“This means that there is very little if any power to be found from removing the air filter, much less changing the filter type.”
“Well there is a clear pattern on filtration ability compared to both flow and the type of filtration media used. The "high performance" cotton gauze and foam filters do not filter as well as some have claimed. I actually received an e-mail from K&N stating their filters filter within 99% of the OEM filters. This may be true, and 1% may not sound like much. I contend that 1% over many miles, may be important. “
“You have to decide for yourself whether you value ever last ounce of power or filtration. I cannot, nor will I make this decision for you. I do know that on a relatively stock car with a properly sized air filter, you indeed have very little if any performance to gain by swapping filtration material.”
“I personally do not like the difficulties associated with servicing foam filters. I do enjoy the simple drop in of an OEM style filter. Imagine servicing and changing 5+ filters in one day for pressure drop testing. You begin to dislike serviceable filters quickly.”