Casey Spencer actually did 550 miles and ended at the starting point (if the comments below the article on teslarati.com are correct, because I couldn't find exact details on the route myself), so the initial downhill segment doesn't matter. But he did deactivate or turn down as much as possible everything that used even the smallest amount of power (climate control, radio, dimmed screens), overinflated the tires, and chose a favorable weather pattern with a strong tailwind in the last leg. And of course he only did 24 mph.
Anyway, I came here to write exactly what you wrote: Elon said that someone already did 800 km, so they should be able to break 1000 km in 2017 or so. To me it certainly sounded like he was talking about a similar hypermiling record, not real world performance. And since the record was done with an 85D while 90D is already available, they ought to already be able to achieve 937 km today. So it looks like they won't improve much in the coming years, 5 to 10% per year like he said.