Sounds like you have alot of searching to do, the sparknotes version is that there isn't a flat out "best" downpipe or intercooler out there, there is significant aftermarket support for our cars and many companies produce excellent product. Search around and you'll find numerous threads with reviews of all these various options, and are you talking about the Okada coils? They're very expensive indeed, and the jury is still out on them. Enrita and a few other high WHP E9x guys have been testing them out, i'm not sure what they concluded about 'em.
E88 N54 Alpinweiss/Coral Red/Motiv HTA 3586r Tial .82AR/Meisterschaft Race/Berk Resonated/ER CP/Synapse/VRSF FMIC/FFTEC LPFP/HFS-4 DO Stainless Lines/BMS Thermostat Delete/Rob Beck PCV Valve + Cap/BMS OCC/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