When I was shopping for 4-door coupes (panamera, 4door GT, and 8 series), I drove both the M8 and M850i, the only tangible difference I felt on the test drive with a bit of highway and street is that the M8 is considerably harsher even on "comfort"...
And the M8 dont' have the nice looking glass shifter (and no option for it)...
I think the Mx50i series are what Mercedes now called "AMG Tuned" 43/53s?
I mean I tried the 840i too, and right off the bat, the 850i was massively more "push" off the line than the 840i... but between the 850i and 8, I couldn't tell tbh...
tbh, BMW M companies build the car off the same assembly lines as normal BMW cars don't they? Last time I checked, the M5/M6 were build in Dingolfing Germany, same place where the 8 series are built.