[DuskStar]

> But x2 fuel consumption is a hard lower bound.

Can't you gain a bit of that back by flying higher than a subsonic design could?

[ddalex]

Lift and drag are proportional. You fly higher you need to go faster as to get the same lift, and you'll encounter the same drag.

[DuskStar]

That's only true for induced drag, though. Parasitic drag is independent of lift, and would be reduced by flying higher.