I have an 07 5.9 w/ 4spd auto. 2500 HD. I have 24,000 miles now and it has been perfect. I live at 7800' rocky mountains and it performed great all 4 seasons. Pull 12,000# over two 10,000' passes very often.
I have two friends with the 6.7 auto and they have been great. Both of them are over 50,000 miles and they pull 38 and 40' Montana's with double pull two atv's behind the 5th wheels. No problems at all. The new set up and new auto is just plain awesome.
My son just bought a new Dodge long bed with tow package etc etc a few months ago. $32,000 what a deal. He has 7000 miles now pulling a 5th wheel tow hauler 38' 5th wheel.
The Turbo set up, used as a brake works flawless so far and is very effective.
The 6.7 gets less mileage than my 5.9 but the 6.7 dumps that fuel in to clear the trap so that's just the way it is.
That's my .02.
Good luck