Reporting in from Mammoth Lakes. Made the drive today and everything worked out great. Now I feel silly for worrying about it. Stopped in Groveland and took a leisurely lunch (the fries at the Iron Door Saloon are toddler-approved btw...), and when we got back to the car it was at 99% so we packed everybody up and hit the road.
The one disconcerting thing is that the nav would never commit to the route even being doable. It didn't seem to acknowledge that the core section of Tioga was even a road. This is possibly due to the lack of cell service up there or the fact that the road is closed for half the year. For about an hour the nav on the driver display looked like an old school game of asteroids... just a pointer on a grid. No matter, great views through the passes and hardly any turns so no need to look at the nav. Once we got to 395 cell service came back and the nav became sane again.
Anyway, until you get over the pass to 395 speed is pretty limited just because there are no passing lanes, but once we got to 395 it showed just how much ample energy we had to work with so we finished the last leg... briskly. Got to the Mammoth Lakes supercharger with 28% left. Charging past 90% was totally overkill. Granted the weather was perfect... For the record this is an X75D with 20" wheels and 2500 miles on the odometer.
So to answer the question "Can Model X 75D do Groveland SC to Mammoth Lakes over Tioga", the answer is "dear God yes, do it and have a lovely time".
Thanks so much to everyone that encouraged me to take the trip.