Sounds reasonable. I would hesitate to offer specific velocities without specific data .. but, then again, that might be just because I am a boring old conservative engineer.
Re 1000kt, making the usual assumptions, the normal sonic calculation is
a1/a2 = square root(T1/T2)
with the temperatures measured in degrees absolute.
For the example you quoted
a1 = 661 x square root(700/288)
= 1030 kt
or thereabouts ....