center of pressure
I looked up my Viper chart for movement of the c.p. as we went from 0.9 M to 1.4 M.
We were not static stable until 0.95M, but went from a c.p. of 43% MAC at 1.1M or so to almost 60% MAC at 1.4M, and then it leveled off.
So looks like 1.4M is a good rule of thumb for less concerns about changes in center of pressure. Have to look at Concorde and Blackbird stuff to confirm.
My guess is the pneumatic system allowed the feathers to move, unlike a hydraulic system ( compressible versus not-so-compressible acting fluids). So the shock waves and movement of the c.p. seems to be an area of concern.