My experience with a jackplate showed some tradeoffs. Lifting the motor up will decrease motor drag, but lessen the effectiveness of the trim. Trimmed boats have less hull in the water, and therefore that increases speed. Decreased motor drag should increase speed also, however, you still need to have enough trim to reduce the amount of hull in the water.
So the best height for the motor on the jackplate should be the highest that allows enough trim up to maximize speed. The only way I know to find this is by trial and error with a speedo or GPS.
Another thing. A motor sitting high on the jackplate will need to be trimmed all the way down to have it get on plane. That setup is fine for the owner who drives it all the time, however, a guest driver may not understand it.
I would start with the motor's anti-ventilation plate adjusted 2-3 inches above the hull bottom. Make a high speed run with max effective trim and note speed. Raise the motor 1/4" and repeat, until speed is max. Now see how she drives under everday use, and adjust motor down to make her more driver friendly.
More notes. Make sure she is getting enough cooling water when adjusted high on the transom. Also,A-V plate will often be out of the water, lessening the effectiveness of the zinc anode fin to counteract prop torque. That can be addressed.
