Usually 2 things can cause high speed overheating - inadequate water flow through the motor and/or aerated water in the motor due to a disturbed water flow into the inlet screens. Most often that is due to transducers or fittings mounted too close to the motor centerline or thru-hull fittings or hull strakes on the bottom of the boat inline with the motor.
If the water pump kit was a factory part, not a different brand, and the correct full kit was installed properly, temporarily install a water pressure gauge and record the readings at 1000 rpm intervals plus idle in gear and full throttle. Compare with the factory specs.
For aerated water confirmation, a clear hose from the engine block to the EMM will show air bubbles or turbulent water flow through it.
In some cases after finding broken impeller blades, parts of the rubber vanes could enter the cooling system and engine block restricting water flow.
"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.