Final report: Lowering the motor back to the original (lowest) position and replacing the 15 pitch prop with a 12 gave me 5100 rpm, a 3 mph increase and full steering control with no cavitation in turns. All is well.
This is what we need to see a photo of. Tilt the engine down and get the anti-cavitation plate level with the bottom of the boat, the get down and take the photo level with the anti-cavitation plate.
