Performance would be one issue... Delta Golf (and 01, who went to Mach 2.23) basically "ran out of steam" at that speed.
Just wondering was that the maximum speed "in" the design ? I understand that "the higher & the colder = the faster" was the key to the performance and that the Mach +/- 2.0 cruise was implied by limiting altitude to FL 600 in order to mitigate cabin depressurization consequences. I guess there where also thermal issues but was, say, Mach 2.2 @ FL700 "warmer" than Mach 2.0 @ FL600 ?
Also wondering what was the max altitude ? Was high altitude stall (for the lack of a better word) ever experimented during tests or training ?