I've been pulling large 5th wheel trailers with my mega dually for over 6 years now without issue. I don't have the factory puck system, but did install the Reese slider that is their own puck system. I have never actually used the slider yet, never had a need to.
As for the mega cab, I don't think I could buy a truck again without it. Its almost a must have if you are putting adults in the back seat. So much more leg room than a standard quad cab. And the extra room behind the rear seat is fantastic. I can put suitcases back there.