"Actually, if you use proper plugs they come pre gapped. The NKG BRK6EK plugs for the M50/M52 motors are pre gapped. " that is just like saying the wheels were all torqued properly and in a pattern, it is always good to double check. mechanics and factory workers make mistakes, I am a mechanic and I make mistakes. Finally the only plugs that get gapped are two or more pronged plugs.
On a side note K.I.S.S.( keep it simple stupid). there are four things that and engine needs to run Air, fuel, compression, spark. it is most likely not air, maybe fuel( injector), maybe compression, if spark change all of the spark plugs and remember that bmws are very smart they remember how things work and they remember how they and the coils work. you may need to reset computer. try a compression test( remember to take all of the spark plugs for no compression leak) or testing to coil packs. if the compression steadily rises then it may be the rings if that is the case or the compression take a few drops of 10w-40 and put it in the low compression engine. if the compression goes back to normal, then I pray for you and the rings are bad. if not then compression leak down test. I could go on for hours, if you need more pm me and I can help I have 4 ASE certifications and about to get a fifth. I would to love to help, cars are my passion.