A peak Mach number of 3.15 at 110,000 feet for the SR-71 is what one normally hears during hallway gossip. If you calculate the leading edge stagnation point temperature for that condition, you get the melting point for titanium minus a reasonable safety factor. Experience with the SR-71 was it flew more efficiently the faster it went (it flew in ramjet mode once its inlets were started). However the melting point of its leading edges set the maximum Mach number placard even though it was burning kerosene more efficiently.

The gossip / "urban legend" about Aurora is it was a drone built as a prototype. However the prototype was found too expensive to operate and the project mothballed. It would be possible to take SR-71 technology (variable cycle ramjets) up to Mach 6 if the leading edges were cooled. However such a bird would be frightfully expensive to manufacture (all titanium with lots of fiddly plumbing).

Gary