An interesting question. I think Vc10
is spot on subsonic, and for what they are worth, my estimates are:
Unrestricted: JFK-LAX 2h 05m airborne / 2h 30m schedule
USA speeds: JFK-LAX 4h 20m airborne / 4h 45m schedule
Not a bad guess, but 250 kts up to FL 570 would be much too slow for Concorde at altitude - as I think you suspected.
The lowest authorised speed (VLA) is 250 kts up to FL410 and 300 kts thereafter, however even these speeds are far below the optimum climb speeds.
If we assume no speed restrictions whatsoever - a big assumption - Concorde’s optimum climb profile would be to achieve 400kts IAS by 5,000 ft, climb at that IAS to around FL320, accelerate to reach 530kts IAS by FL440, climb at that IAS until achieving Mach 2.0, at around FL510, which it would then maintain, in a cruise climb, up to FL600.
Again, I wouldn’t disagree with your estimate, but the maximum cruise Mach number for Concorde is Mach 2.0, and Concorde has never done 2h 31m across the Atlantic.
To date, the fastest Atlantic crossing achieved by Concorde, was on the 07 Feb 1996, Capt Les Scott, JFK-LHR, 2h 52m 59s.
Having talked to the crew about the unusually favourable conditions pertaining on that day, the winds and temperatures, the runways in use, and, perhaps most of all, the invaluable help from ATC on both sides of the pond, this is probably unbeatable.