This quote made by you says it all.......
"I decided to lower the motor on the plate about 1/2" and see what happened. Once I did the water pressure immediately rose and was riding around 20+ through most of the speed range. I could run full throttle and as much trim needed (even over trimmed for optimal handling) with 15-20 psi"
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You lowered the motor a 1/2" and had plenty of water pressure, with FULL trim. This proves that you are sucking air. If it was a blockage problem the pressure would not have increased, when the motor was lowered.