Turtlehead, yes that's correct, hence the Boeing requirement (737, at least) to slow down at lower levels to account for the reduced impact tolerance. They are, however, in no way "unstable" and there are no limitations at upper altitudes without window heat. Now, if you are correct about the Airbus windows not requiring heat to maintain their integrity, I go back to my query about how there is anything "unstable" about them in such circumstances?