On the original subject, I did a little data gathering at work today.
Unfortunatly the red / black boxes are not always visible, being off the bottom of the scale in the cruise. I could however record the position of the yellow band which represents a 1.3 g manoeuver capability:
Fl 380 yellow band at 226 KCAS = 215 KEAS = M 0.72
FL 80 yellow band at 178 KCAS = 177 KEAS = M 0.31
I know weight effects it, but there was only 0.8% weight difference between the two readings.
So in EAS terms there was a about a 21% difference in the position of the yellow band between FL80 and FL 380.
In CAS terms the difference was about 27%