I´ve some doubts about it. I´m not quite sure how Boeing wants to manage the certification process as they would have the Sonic Cruiser to fly Mach 1.01-1.04 to get it certified to Mach 0.95-0.98. That would mean a different design.
And the choice of engines will be a question: all manufacturers have back away from developing derivates of the current T7 powerplants and fix on new designs instead. That means that this site will contribute another share of development costs to an already not cheap plane. And will all three manufacturers be able to sell their engines in the long-term? The A330 and B777 projects were a desaster till today because three manufacturers have to share the whole market and two of them came out with a completely new design and the other had to build a derivate which comes close to a new engine.
And what about the ecologic issues? Flying in this heigth contributes further to the green house effect - much stronger than in the lower atmosphere. And the increased fuel burn itself will provided more NOx, CO etc per pax compared to a B744 or A388.
Just some food for thinking...