There are many other steps you should make before adding wedge to your pump. If you don't have a Place Diverter, I would suggest getting one. The will help you get more of the boat free'd up. IF your still not happy, you could get a cut back shoe, and ride plate combo. With these, you should be able to free up the boat for more mph and still maintain a smooth ride (no porpoise) I would also look into what impeller you have, and find out what RPM makes the most power for your motor. IF your impeller it too big, you could have it turned down (example, AA to and A)
Without knowing the condition of the pump, proper impeller, condition of the motor and hull, I would say 55mph is about right for a 21ft boat.
HBjet