When you say, “power reduced to an idle,” do you mean YOU reduced it to an idle or the engine dropped itself to an idle? There is no “Guardian” or “SLOW” system, so if it dropped ITSELF to an idle, you’ve got problems. Hopefully it wasn’t about to seize. Check the plugs for water, the sign of maybe a blown head gasket. Otherwise, I would pull the lower unit back off and inspect the pump again. Maybe something came loose after an hour’s run. Don’t forget to attach a snake wire or string to the shift wires, before to pull off the lower unit! That will make it easier to put back on.

Well, I think your plastic bag theory is a very good possibility, happens all the time…Once you stop and tilt the engine up, the bag has already dropped away…
And yes, unfortunately, these warning horns usually don’t go off until “it is too late” when running at high speeds. The engines heat up almost instantaneously when something goes wrong. You mention that the powerhead does not show signs of overheat such as burnt paint/melted wiring….I would definitely retorque the heads and exhaust manifold though, especially if you had them off during your repair.
I would pull the gearcase off though, just to make sure nothing has gone wrong. Look for places where the water pump could have sucked air once the boat was up on plane and the water level dropped below the SS impeller plate. You mention replacing the impeller, did you use an OEM piece? Check to make sure the two water pipes are properly aligned and into the grommets properly. Don’t forget those two white plastic tube guides that are usually stuck up on the tubes that will help with realignment.
And, like Bill said, don’t forget to attach a snake to the shift cable before pulling the gearcase down…

I’m not saying remove the bolts, just torque them, and snug up the exhaust cover screws, you will have to remove the ignition bracket to get at them though…
I don’t trust the aftermarket impellers, heard too many negative things. Does your engine have its original aluminum impeller housing, or has it been replaced by the plastic one with the SS liner. The plastic one has a spaghetti seal that seals it to the impeller housing, make sure that is in place.
Your temp specs seem correct.
The overhead occured at high speed, correct? Your latest post almost makes it seem like occured at slow speeds.
I have never heard of one of these engines getting “air locked” cooling wise, but I guess anything is possible.
The paint didn’t get burnt/discolored, and no melted wiring, correct?
I wouldn’t waste time and money with a temp gage. The same scenario occurs, the engine starts to bog down, then you look down to see the gage pegged. Plus, you have to add an external wire to the harness for the temp gage sender up to the gage on the dash.