The 727 actually cruised faster than the Mirage.
I was taxiing out in DRW from the old terminal when a 2 seat Mirage taxied from the ramp north of the water tower. Couldn't hear his destination (he was on UHF) but think it was Mt Isa, the tower sounded very pleased when they told the pointy nose to hold and let faster proceeding traffic on the same route pass.
The Pointy nose queried this and was informed he had planned at Mach .8 and the 727 at .82.
I once took an empty 727-200 MEL - SYD and used the Mach limit switch to change to the higher Mmo limit and cruised at m0.89.
Those were the days of no speed limits.