In my opinion the 650i Gran Coupe x-drive (+ M Sport) is the car from BMW to be put near Panamera GTS!
Some comparos are very bad and the conclusions are even worst so sometimes you wonder what was the initial point for doing it?!
I woudn't buy an M5 over a 650i GC if I'm not interested in tracking my car.
I still think that the natural habitat for the M cars is the track