Got this from another thread here:

Quote Originally Posted by jplazo
Before buying a K&N and other overpriced filters, check this out:

http://bobistheoilguy.com/airfilter/airtest1.htm

K&N attempted to counter the findings... but with contradictory statements.

The bottomline is that the stock filter does the BEST filtration and with minimal pressure drop. Branded after markets do lousier filtration with 2 to 8% more airflow.

For stock cars, this sneeze of extra air isn't worth the extra expense.