There's no doubt about K&N's air filter sucking (pun intended)...
Here's a fun experiment for someone: Buy a differential manometer (hint: try searching ebay for magnehelic
). Now connect the filter to a vacuum and connect the hose to a nipple in that pipe. Now build a box around the filter and give it an inlet pipe the same size as the outlet. Add a nipple and connect the other gauge hose to that nipple... now turn on the vacuum or engine and record the pressure drop across the filter. Now try the next filter and record it, and so on. This and the minimum particulate size are the two important qualities behind a good filter... personally I'll take something with a tiny bit more restriction if my engine will last longer, but with the three I listed the pressure drop I've seen reported is minimal anyway.