Bill
I thought the REASON for over heating due to no or low pressure in the cooling system was the "boiling point" factor. IF the water boils at 212 (which it WILL with NO pressure) water will be pushed out of the system. THEN you will be LOW on water, as a result, over heating or higher than normal temp.
The OTHER possibility is that the "lower" water temp (ONLY 212 or lower) does allow as much "heat transfer" as a higher water temp would allow for?
Water temp at 240 Deg F sheds more "BTUS" than water temp at 211 Deg F. Not sure my idea of the thermodynamics postulated here "holds water"........
Ernie