Keep in mind K&N recommends replacing the filter after 25 washings. My Stang sees only highway driving in non dusty conditions, so they last a long long time. Independent tests show they do flow better than a paper filter, but they also don't filter as well as a paper filter. In dusty conditions [like with my truck] I don't use them.
I test all performance upgrades that I do on my Stang with my G-tech Comp., the K&N drop-in filter had 0 effect on my 1/4 mile times, it may have made a small HP increase, say 1-2 HP, but was not measurable in the 1/4 times.But hey, if you find, say, 2 or 3 other little mods that give you a 2-3HP increase each, added together will yeild a noticable increase. As they say, every little bit helps.