well...after about 300 miles on my new K&N...just saw some increased mpg (avg 21.5 to 23)...and no, I wasn't pinned back in my seat from all the added HP the filter added...

 

Interesting read on K&N filter vs oem BMW paper filter. This lab-conducted study showed some very interesting results.

http://www.seansa4page.com/resource/airfilter.html

I suppose it depends how well the oem filter is designed. My Audi's oem paper filter is much like the M3 filter tested in the article. There are very many deep pleats, thus providing a huge surface area compared to the typical aftermarket filter. But on other cars, it's possible that the oem paper filter has far less surface area.

Interesting read, either way.