Thursday - last edited Thursday
Your top rpm is at the very bottom of the recommended operating range - 5400 to 6000 rpm.
Going down to the 16" RX-4 will bring the revs up, greatly enhance your acceleration, improve the hole shot, but may lose a couple of mph.. Propping is mostly trial and error.
Another suggestion is to try the new RX-3 style which came out this year in a 17" pitch. The 1" reduced pitch should put your motor in the 5600-5800 rpm sweet spot.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.