I can tell you that the high wattage freeze plug heaters do a heckuva job. I had one that was installed at the factory on a '78 olds 350 diesel, and it kept the coolant so warm that I had heat from the heater core instantly upon start-up. There was no circulation other than by convection within the engine block and it did not get hot enough to open the thermostat, but came pretty close. As I recall, I had a mechanical temperature gauge on the intake, and the heater kept the coolant above 150F if it was plugged in all night. When it was parked in the driveway and plugged in, snow would melt on top of the hood above the engine.