In order to make changes successfully, you need to understand how the engine and hull works through the water. The "cavitation plate" incorrectly named is really an anti-ventilation plate. Its purpose is to provide a physical barrier so the top of the prop does not suck in air due to the negative pressure behind the prop blades. NOW: when the boat is on plane it does not really ride flat on the surface of the water. In fact, it creates a shallow vee shaped hole in the water which is rapidly refilled when the transom passes. Since the transom is lower in the water than the surface, when the water rushes back in, the level is higher than the transom. THUS, if the anti-ventilation plate is below the transom, it is deeper in the water than it needs to be..Extra lower unit will be dragged through the water increasing drag and lowering top speed.
Depending upon the hull design and set-back of the engine, the engine can be raised on the transom up to several inches putting the anti-ventilation plate AT the surface of the water when at plane. This will increase speed but not by a magic number. You need to experiment with different props and engine height settings to achieve best results. ONY change one thing at a time so you can compare results.
Usually, higher pitch will affect hole shot negatively so some high pitch props are vented. This means a controlled amount of exhaust is bled through holes in the hub and ventilates the prop, allowing the engine to rev up easier. Water flow over these holes at speed effectively closes them.
