I have always had a CAI from K&N on all my past and present cars with good performance...except my BMW.
K&N does extensive testing on their products and could not make power on a 550 engine!
Not sure of other models but therefore just a drop in. I have the BMC and RPI ram air which they (RPI) dyno'd
and made power with and I can honestly say it sounds and feels more responsive!
Good luck!

 

I realize this is a veryyyy old thread but I just stumbled across this on a e90 forum. Very interesting indeed

http://www.e90post.com/forums/showthread.php?t=137667