Leave the OE box be or get a dci type intake, IMO drop ins are a complete waste of money. That being said, until you start pushing over 16psi or so, a dci really doesn't do squat. People are running sub 12 quarter miles with the stock box, an intake shouldn't be a primary concern. Mine was free, or i'd still be using the stock box.
E88 N54 Alpinweiss/Coral Red/Motiv HTA 3586R/Meisterschaft Race/Berk Resonated/Macht Schnell 3"/ER CP/Synapse/VRSF FMIC/FFTEC LPFP/HFS-4 DO Stainless Lines/BMS DCI/BMS Thermostat Delete/Rob Beck PCV Valve + Cap/BMS OCC/PTF E50/ST Coilovers/M3 FCA + Tension Rods/M3 Subframe Bushings/M3 FSB/DINAN Camber Plates/Apex ARC-8/Project Kics/VAC Hubs/Rogue Transmission Mounts/Alpina TCU Flash/Icarbon/Kerscher/BMW Performance