http://www.knfilters.com

In their "facts" section they've got links to two independent lab tests showing airflow comparison charts. The K&N flows considerably more air than most stock filter elements.

I'll see if I can dig around the stratus board a bit and look for some dyno charts. I know we've got a couple floating around somewhere where someone had dyno'd their car with nothing more than a k&n filter. It showed like a 4whp gain.