The only advantage of M1.3 is that you can get away without fancy intake geometry. Apart from that it is still right in the transonic drag rise. Once you've sorted the intake it is all downhill because the transonic drag reduces and the intake pressure recovery does lots of the engine's work for it. Personally I reckon they would be better off trying to achieve a high subsonic cruise Mach number, say 0.95 or 0.96. That would get you there ahead of the opposition with minimal extra fuel consumption. Otherwise the next workable solution is M2+ using lots of titanium structure for the hot bits, but that idea proved to be a non-starter because they couldn't get the takeoff noise down to acceptable levels.