Happy to report that getting around with the Bowlus continues to be easier than I thought. Yesterday I drove Rocklin to Truckee, which involves a 7000' elevation gain climbing up Donner Pass (Donner Pass to Truckee is downhill and you can do it on regen alone). It took 190 miles of range driving at 55 mph with climate control off. The temperature was 50s and I have 20" winter tires filled to 46 psi (the Bowlus tires are at 50 psi).
I'm also finding myself driving 65 mph in places where the Supercharger density is high (note: in California there's a 55 mph limit for cars towing trailers). If there isn't much wind and the elevation is relatively flat, the energy draw is usually 550-600 Wh/mile at that speed. You do get some sway if there's a crosswind, but the Model X has anti-sway built into the traction control, and it seems to work pretty well. Another interesting thing is the trip planner has gotten to know the trailer characteristics pretty well over the last 1000 miles, and it now does a good job of projecting the battery range (you can toggle in and out of trailer mode to see the projected impact of the trailer).