Realistically, I think with G force, comfort levels and crosswinds etc. I use the speed of the Concorde which was roughly half the time of a regular flight.
Plus I like to say in my world, a design like the Concorde returned bc its too cool not to. Seriously - if you study the design, structure, operating procedures and so much else, it was far ahead of it's time. I take all the specs and better them a bit for SR4 - cargo capacity......