I tow around 7,000-8,000 and have been able to keep it well over that speed, but then I don't have a box trailer (anymore). The only time I have had any need for tow/haul mode is when crossing the passes in the Rockies. The rest of the time, I just leave it in normal mode and it seems just fine. It sounds like you are doing the same.
When going through the Rockies, cars would go around me to get a jump on me before we hit the passes. Where there were two lanes (passing lane) to work with, they were quite surprised to see me going back around them when the climbing got tough. In switchbacks, I was stuck with having to wait behind the slower moving cars that had passed me beforehand.
I was used to this in my previous tow rig (2006 Dodge with Cummins, 6 speed manual, and 4.10 gears), but was quite pleased to have a similar capability in the new rig. It should only get better as we get over 20,000-25,000 miles and the engine starts getting broken in.
