I don't think there is any middle ground. Aircraft are already flying at roughly Mach .85 (so 85% of the speed of sound). So there isn't really any middle ground they can use without simply jumping properly into SST territory. The sort of time saving which could be gained from a small increase in speed remaining below subsonic would be completely negated by favorable \ unfavorable winds.
I can't really say what the environmental credentials of SST travel truly is. I do know at subsonic speeds a SST aircraft is terrible, but I believe at supersonic speeds it's not too bad. That said, the version of not too bad when concorde was first built and the current day view of not too bad probably differ greatly.
Agreed...
