I pull a 40' fithwheel with a 2015 6.7 F350 and it pulls like a freight train, my only issue with my truck is the small fuel tank however comfort and torque makes up the difference.
I couldn't decide between the m8 or The pro Rmk, but after doing some research on the internet I decided to go with the m8 ... It seems to be the more reliable sled of the two.