Interesting point about the plate and rudder control Gman. I have noticed on my friends 19' I/O on the Sound that control of direction is certainly reduced while using the troll plate. If it is indeed just the motor, then getting a new prop would probably be the cheapest and easiest way to get the speed under control.
For conversation sake, say that if one did fabricate a troll plate, what if it were made in a V pattern instead of a flat plate. By my thoughts, the V pattern (pending angle) would split the prop wash and instead of completely obstructing it, directing some out to each side which theoretically would slow the boat a bit, but still allow for more forward movement and control than when using a flat plate. One could even make the V angle adjustable via a hinged plate I'd think.