If high stage valve (or what ever each manufacturer calls it) is stuck open at high thrust it will cause relief valve to open and dump surplus air, also pressure regulating / shut off valve will move to close due to over heat of the air flow. If stuck closed then as stated by mudukace then no real effect except on the ground at idle......pack flow low etc. and of course at top of descent when lack of flow / pressure will show as possible difficulty in maintaining cabin altitude and on some types as you continue descent possible lack of thermal anti icing......