Congrats on the diesel. I am pulling my 210 Sea Ray Sundeck with my diesel and I can even get 15-16 mpg going on flat with 65mph. The Sea Ray is around 5k lbs with the trailer, but is twice as wide and twice as tall than the X5 so the aerodynamic resistance over 60 mph puts more strain that the weight. I can still go our very steep and long Tejon pass with 60-65mph on 6th with 1 or 2 short downshifts to 5th.
These 6 and 8 speed transmissions work like manual ones-the torque converter is locked by default and it will be unlocked only for a brief moments when the engine is out of boost(torque). Once the car is under way it will never unlock, but downshift instead. No TQ slip=no heat in the tranny so this should not be any concern at all.
My '03 4.6is was unlocking the TQ at any sign of uphill and the transmission was usually hot halfway up in a hot day.