I haven't ever heard good things about K&N coming from working on cars for a living and riding sport bikes. I have "heard" of a few power losses on bikes. As for cars, yes, I have seen them mess up maf's from being over oiled.
For air filters, I'll always go oem unless I have an actual aftermarket intake on the car.
I've got the itg Maxogen intake and have had no problems with it.
'05 Elise #557, CO, Touring & Sport pack, both tops, B&M short shift, itg Maxogen intake, Racing harness bar, RTD2 brace, Fire Extinguisher, CF console, Battery extender, Lidbone, Bootie
'61 Land Rover Series II 109 3 door--sold
'66 TR-4A IRS--sold
'71 Porsche 914/4--sold