but I'd guess that some of the technologies that make SS travel possible may make subsonic travel more efficient.
I wouldn't count on that. if a plane is optimized for efficiency during the longest part of its flight, which would presumably be supersonic cruise at high altitude, a bunch of things that make modern airliners as efficient as they are (blended winglets, and turbofan engines where the fan generates more thrust than the actual turbine engine exhaust) are directly counter to that
It's an entirely different kind of flying. All together!