I had this problem last year but it did not matter what speed. My dealer replaced the impeller and it still overheated. After some searching they found small pebbles and vegetation inside the cooling system that had to be cleaned out. On my 2007 270 the thru hull fitting for the engine does not have any type of screen protecting it. I have called Monterey on it and asked what can be done but really did not get a good answer. I had asked about a sea strainer inline but the response was no due to water flow needed for the engine. I guess if I would have done it, it would of violated my engine warranty.
Two years ago I restored a 1995 Monterey 276 that was in bad shape, after two replacing power (new 4.3s) and outdrives on my shakedown cruise the port side 4.3 (from that date known as the bad twin) started to overheat. During my investigation to the cause of the overheating on this brand new equipment I learned a few things, one of those might apply here: The pebbles inside the engine could have cause damage to the water pump and could cause insufficient cooling of the engine. Also check the thermostat to make sure that a pebble isn‚Äôt keeping the thermostat closed. I am curious though. I may be showing my ignorance here but why is your engine cooled with a water source the come in from a through hull?
Finally the source of my own overheating was a defective impeller. The compound used to glue the impeller to the bearing was either misapplied or not there at all. It caused the impeller so work at low speeds (I assume because of friction) but that at high speeds the impeller would heat up and begin to slip so much that it did not move any water.
Hope my armature information helps.