I had an M800 in last week to check for poor idling which turned out to be closing clearance related. Well, I expect anyway, the owner was going to play with it.
But one of the things I checked was voltage at the coils. The horizontal coil is easy to get to, just under the back of the airbox. With the battery at 13V or so, the coil had 11.5. Jumping battery voltage direct to the coil (and therefore into the rest of the little loom that is downstream of the injection relay: coils and injectors mainly) didn't help the idle quality, but richened the mixture from 2.5 to 4.5% CO without any other change.
I have had another M400ie with a similar voltage drop issue, but i don't recall checking the idle mixture at the same time as playing with voltage. It had an ongoing misfire issue (on the horizontal from memory) that I never got to resolve to my or the owner's satisfaction before he got sick of me and went elsewhere. But I'm quite convinced it was related to voltage drop in this area.