mtofell1 I tow TTs & boats daily and in the mountains commercially with the 8 speed. Its going to be a game changer for towing with the 6.4 HD. Should even make a perhaps not great but measurable difference in mileage and acceleration.
I've towed 15,800 CVW up and down 6 percent grades. None of what you describe. I have a tuned Ecodiesel so only about 270 HP & 465 TQ IE less power than the 6.4 and less weight than you are towing but in the ballpark and should give you some comparison.
My understanding is that you have pretty good compression braking with the 6.4 & 68RFE. The ZF has a trans brake as well and works real well with the exhaust brake so I would assume it will work well with the compression braking and you will not lose that either. It sure is nice coming down a grade with weight and having no worries about keeping speed under control, heating the truck brakes or being able to stop.