K&N gave me a slight improvement in throttle response and gave me the highest "stock" figure at a JKM RR day (by 7 bhp) so I'd suggest there is a few bhp in it too!
There is a K&N demonstration at my local garage which has a typical OEM paper filter, and a K&N panel above an electric fan. There is a big difference in airflow through the K&N which you can feel with the palm of your hand. There is no doubt in my mind that the K&N gives a better flow
They just need a good wipe out of the box to remove any excess oil.