Most BMWs have nice cold ram-air box, K&N drop in should do the trick. Sometimes open intake design just suck up hot air, making the mod counter-productive.
I had dual Pop Charger Intakes when I had my Z, it didn't make any more power on the dyno with the hood closed + hot engine compared to the stock box. It just sounds good. Most people dyno their new intake upgrade with the hood open.