What will happen to the gyroscopic instruments (heading indicator and attitude indicator) if the vacuum pressure is too high? Will they be sensitive or stable?
It means the intake filter at the other end of the air system is clogged or otherwise obstructed, or some other obstruction in the plumbing that the vacuum regulator is unable to compensate for. The high suction is an indication that the flow upstream of the tap-in for the vacuum gauge is restricted.
The effect will be that the velocity of the air jets driving the gyros will be too low, and as a result they may give inaccurate or erroneous indications, have too much precession, or maybe just tumble on you. Bottom line is vacuum above or below the green arc means unreliable indications, from the gyro speed being below specification.