In most reasonably scientific tests I've seen K&N filters marginally outperform most, but not all, cellulose filters in terms of flow restiction, but air filters are a very minor restiction to air flow unless the filter is undersized for the air flow capcity of the engine. For most applications the difference in peak power is within the range of normal test error - one to two percent.
New K&N filters supposedly pass the SAE J726 filtration test - their ability to trap particles, but their filter efficiency is highly sensitive to oil content and the amount of oil in the filter decreases with time/miles, so at some point it's filtration efficiency becomes poor, and they will pass a lot of small particles.
I find the "cleanability" of K&Ns ludicrous. Would you wash out an oil filter in solvent and reinstall it?
Bottom line IMO is that they are overhyped by aggressive marketing/advertising, not worth half the price, and their filtration quality is suspect You can buy three to six quality cellulose filters for the price of the same K&N depending on the specific filter. At 30K miles average recommended change intervals you are money ahead to use a qualilty cellulose filter and change it twice as often. A cellulose filter can clog and cause a loss of performance, but as they age they don't pass more dirt.