something you may not need, check out this thread from a few days back. Theories on this vary.
http://www.bimmer.info/forum/showthread.php?p=42588
I think the that the K&N air filter in the stock box works just as good as the cone as long as you keep the K&N clean, and lubricate properly and periodically. Also, the stock air box has a heat shield right in front of the front exhaust manifold. Mine is equipped with a heat shield that is painted black... bad idea. I'm in the process of stripping it and will paint the shield metallic so that it reflects the heat away from the box, not into the box as would be the case with a black shield.