.07 margin it is. Too much late night typing!

One more clarification to make is that, although the airplane could go supersonic and did according to test pilots, its max speed in a reliable sort of way was only M.99. Subtract the M.07 margin for certification and you have M.92, the published top-speed for the Ten. Supersonic was only possible in a controlled dive. It is technically a supersonic airplane by a little bit, but it just doesn't have thrust to do it reliably for reasons mentioned by Phyler and is not designed to do it regularly.

Drag curves rise steeply around M=1 but come down after about M=1.4 as noted by the Prandtl-Glauert Rule. "Breaking the sound barrier" is a colloquial expression from times prior to 1947 when Chuck Yeager had yet to go faster then the speed of sound and collect the flight test data. It turns out to be an apt term because drag rise near and around the speed of sound is very steep, coming down steeply in the S/S range as it went up in high subsonic range.

Supersonic aircraft can cruise efficiently at reasonably low drag levels, but it is hard to design an airplane to get to the supersonic regime in the first place, very few customers are willing to pay the additional cost. There is a steep drag rise around M=.85 hence the cruise speed of most airplanes is somewhat below that figure. To design an airplane that can cruise at high subsonic speeds requires aerodynamic tinkering for a relatively small gain in speed. It can be done, and many airplanes have the requisite drag reduction features to make it possible.

But it is not true that drag stays high in the supersonic regime- it is surprisingly low. Drag comes back down to subsonic levels by about M=1.4. For reference, see John D. Anderson's Aircraft Performance and Design or one of the other good textbooks. As far as fuel burn is concerned, thrust specific fuel consumption is very fairly constant with speed. This explains why the Concord cruised at M=2 and why recent fighter jets are optimized for high speed cruise (F-22/ F-35).