another data point on .77 being SC only. My car is at SC and was updated. FW update time ~2 hours (supposedly AP updates are 2 hours at SC, whereas non-AP are < 10 mins). fixes on .77 include internal temp algorithm update, for better heating, and LTE FW upgrade for better stability of radio stack.
FWIW, It took the service center about 20 minutes to load .77 onto the car.
Then once home I timed how long it took to install. The screen said to expect nearly 2 hours but actual time was 27 minutes.