[Destroyer]

Unless my mind is playing tricks on me I seem to recall somewhere in my years of boating that the waterpump impellers on outboards and I/O's do very little work while on plane.

I think I recall reading somewhere that the force of the boat moving thru the water causes water to be injected through the intake holes in the lower unit and the force is so great that it actually partially bends the impeller out of the way and goes up into the motor by itself. In fact, if you look carefully at the intake holes you will see that they are chamfered in such a way so as to channel the water into the intake at speed. If the intake plate was put on backwards during a cleaning (a common mistake) then the water will not be forced into the motor during high speed running. It will work fine at low speed when the impeller is doing all the work, but not at high speed. In fact, that's the reason the makers of outboards don't use a solid impeller like you would see in a sump pump. The soft rubber can flex and get out of the way at high pressure, thereby making more efficient cooling.

I don't know if this is your problem, but it's a quick easy check. Just look at the intake plate(s) on your lower unit and make sure that the holes are facing forward so that water is scooped up as you travel through it. If they are facing backwards then you just found your high speed cooling problem.