BOAC, I do not think Compressibility, unless you are at the critical mach number will enter the equation here. (Remember that is why we have SWEEP to delay the onset of compressibility) At high and low altitudes, the pressure distribution will in effect move the Aerodynamic center on pressure forward. (Just past the max Lift/Drag curve) At that critical point, flow separation will occur and you will eventually stall. The main thing to know is the difference between IAS, TAS, and EAS. Very simple I know, but the details would take three pages.