I guess M5 ownership means something has to happen every year. Got a misfire a few weeks ago, I just thought hmmm this is odd just got 10 new coils and 10 new spark plugs less than 10k kms. Anyway, I read a little and I found a good potential culprit... Ionic Current Control unit. Fancy pants distributor that BMW experimented on the S85. I thought it doesn't seem to be hard to diagnose so I gave it a try. Swap the left and right unit around, sure enough the misfire just went from cylinder 8 to 3.
The part wasn't cheap, but to reach it was relatively easy. Just remove the airbox, it's cover and wire harness. My daughter helped me a bit, as I have a finger injury. We discussed it a bit, why I think my theory is correct and just order the part. Ordered from FCP Euro with a kind discount from the CEO, about a week after it arrived along with some other parts.
Today my daughter helped me again, my finger is taking ages to heal. Top part is the new one, bottom is the broken one. The other working seems to have been replaced as well by previous owner. I may order another one eventually, ordering 2 will definitely attract customs. Test driven the car, that sorted out the misfire which will appear when car in on temp. I hope the rest of the year will be boring...