I saw where the two possible routes that Inmarsat drew up assumed two different speeds south — one at 417mph and the other at 518mph. That’s quite a difference.
417 knots true airspeed (even assuming knots not MPH, is pretty slow for an airliner at altitude (assuming high altitude and light winds), but I think both speeds are achievable, depending on the altitude. At 35,000 feet the speed of sound is about 573 knots. So 417 knots true airspeed is about .73 mach, a not-unlikely fuel savings cruise speed, and 518 true airspeed is about .9 mach, which is very fast for a 777, maybe even a few knots out of range at 35,000 feet. But at 40,000 feet mach 1.0 is about 598 knots, so that would be about .87 mach, which is likely achievable by the 777 without an overspeed. I think those airspeeds are realistic, depending on the altitude.