I would like to make a comprehensive guide to the available parts for the e9x's. There are going to eventually be infinite modifications to make, but which ones are worth it?

How about we start with intakes -- What is the horsepower gain of each? what is the cost? how does it look? sound? is it out yet?

AFE
Dinan
Gruppe n
K&N filter (drop-in)
Factory air box w/ silencer emoved