We have about 22,000 miles on our 2011 3100SS on a 2010 Ford V-10 chassis. We’re always towing a 4000 pound Nissan Pathfinder. I didn’t notice any difference in power or mileage as the engine broke in. (It had about 975 delivery miles on it when we got it, and I’ll bet the miles were all at 65 mph or more.) The V-10 develops its peak horsepower at a high rpm and the 5 speed transmission’s controller is set up for best mileage. It likes to shift into higher gears but it doesn’t let the engine load up, so it downshifts easily. I don’t think it hurts the engine or trans, but frequent downshifting and engine noise over 4000 rpm is annoying. The situation is worse when using cruise control, since it tries to maintain the speed so closely that it downshifts in the slightest grade. I just control the throttle to let the speed drop a little, trying to keep it from downshifting when I can.
A bad shortcoming is Ford’s use of the old 4 speed shift quadrant, with 1, 2, 3 and drive. It would be very useful to be able to hold it in 4, especially going down hills. The Tow/Haul is very useful for going down hills because when it is selected, with just a touch of the brakes, it will downshift as needed to help hold the speed. Just remember to switch Tow/Haul off if you are coming to a stop because it will continue to downshift into 2, then 1, and it will stand you on your head!