Daytona:

I have seen the issue you talk about too, but when I went further into it, I found it had too do with the water flow through the heads and to the gauge sender. In my case, I took the temp of the water entering the radiator with thermometer, because it didn't make sense. What I found was that the water coming out of the engine wasn't getting more than a couple of degrees hotter at idle, even though the gauge showed it heating more than that. The only explanation I could come up with was that at the low water flows at idle, hotter water coming out of the heads was hitting the sender than when the flow was higher, but by the time it got to the thermostat it had all mixed back together again. Looking at the water passages on the manifold in relation to the head passages, it was very possible that this could happen.

The tests I ran on the TT340 were with the 6 blade AC pump (small diameter impeller, with and without anticavitaion plate) and the non AC 8 blade (big diameter impeller) pumps. I left everything else identical and all 3 setups gave exactly the same temp at idle (running at the .95 ratio). Increasing the airflow dropped the temp to the thermostat.

I have no doubt you saw what you say, but it may have been an oddity, or a bad pump. On all the electric fan engines I have seen, they will run a touch cooler as soon as you speed them up (slug of cold water from radiator) but then heat up quickly to higher than an idle if you let them run long.

I have also seen the results you talk about on engines where the heater has been looped out with a piece of hose. On a marginally cooled engine, this can bypass too much coolant away from the radiator until you speed up the engine and increase the flow. Sometimes even putting a slight restriction in the bypass hose will help, if you are close.

You have to be careful when putting more pump on and engine because of the possibility of cavitation at higher rpms, and the fact that it will cost you hp.

A quick edit: The water flow issue can be pretty easy to test. You can see the flow in the radiator, and you can take the temp of the radiator at the outlet. If you are cool going into the engine, but hot out, you may have a low flow issue. If you are hot going into the engine, you have problem getting the heat out of the radiator, probably airflow.