Originally Posted by BMW_Belgier
bmw engineered the factory intake to function responsibly, efficiently, at 8lb of boost. even the least aggressive tunes turn that up to 12+, and most people are running something between 14 and 17lb. granted this isn't a huge turbo so you're not pulling world-shattering cfm, but there's a chance to outrun the efficiency of the stock filter in that scenario.
the stock setup flows quite well for stock tunes. that's why you probably won't see a big difference just dropping it onto a stock car. remember bmw's design constraints:
-only needs to flow enough to support 300hp
-quiet, because many purchasers prefer -exhaust- noise over engine noise and they've been quite vocal about this over the years
that pretty much rules out an open-element filter. the cone-in-a-box idea is silly from a production standpoint because you then have to tool for that, driving per-unit costs up.
every facet of production is a compromise of some form.
I can't be the only guy who's done his homework on muscle cars AND owned a few turbo vehicles.