M2dude
It was now quite a long time ago, but looking at my log book I see that the flight was about 1hour 35mins chock to chock, so the fuel load would have been quite light. At subsonic speed the cruise height was determined like all other aircarft, in that the lighter you were the higher you could fly, and at those sort of light weights it was possible for Concorde to fly up to a subsonic cruise height of 39,000ft. From there at light weights the supersonic accel could be much quicker than what would be experienced on an Atlantic crossing at heavy weights.
In fact the aircraft would probably be lighter than a round the bay trip where we always had a 100 passengers. One problem with accel just off North Carolina's coast was that there was a north south airway right in our path and there always seemed to be a B757 at 41000ft which delayed our accel until he had passed. A cunning plot I think
Now you might ask if we did not have 100 passengers, then how many did we have between IAD and MIA well it is a secret, but I have to say we were normally lighty loaded. However the trip was very popular with the crews as it gave us a feel of the sun's warmth, which is hard to come by in New York in the winter