632 is right about the MAF too. I forgot to mention that. Visit any Porsche board and search on MAF and K&N to read about the many thousands of dollars spent on replacement MAF elements (at $500-$1000 a throw) due to crud emitted from a K&N filter.

I know it's hard for some people to accept, but BMW engineers are pretty smart people and have done a good job on air intakes in stock configuration. I don't think there's ever been an alteration to air intakes that produce actual documentable, statistically-significant, and repeatable increases in performance. There are a LOT of alterations that screw things up, but it's hard for anyone who's just spent $600 on a fancy brushed anodized aluminum Cold Air Intake to admit that it doesn't work as well as the $20 BMW plastic airbox he just ripped out.