There are lots of people that won't run K&N's on their 1:1's because they let through a lot of fine dust. Other 1:1 guys won't run K&n's because the oil actually comes off the filter and coats the hot wire on the MAF meter.

Here are a couple sites I had saved where they tested a K&n Vs other filters. kinda scary results..

http://www.bmwe34.net/E34main/Upgrade/Air_filter.htm

http://home.usadatanet.net/~jbplock/ISO5011/SPICER.htm

edit.. one interesting way I read to filter out the fine dust that K&N's miss was to buy a cloth shop vac filter and wrap that around the K&N. supposidly it catches all the fine dust.