My experience parallels my northern neighbor. During zero F weather the stock MB block heater is good enough to keep the Benz engine OK if the car is inside the garage. Outside with the nose into the wind, it won't keep up. It simply does not generate enough heat. The last one I put an amp clamp tester on the feed barely pulled more than 4.5 amps. (About 500 watts.) It doesn't circulate the coolant, so at best you have 2 cylinders with a warm spot. The Kats, Hot Shot and other circulating heaters can be found with 1500 watt (& larger) heating elements and they push the heated water through the block.

I'd wish you good luck on the weekend's game, but your guys won't need it. It's about 20F right now, but it's going down. The way our guys have been playing I think Soldier Field will be empty but for Packer fans. Stay warm.

Thermal conduction and thermal siphon should circulate the Coolant heated by the Block Heater.
The only imposition would be the closed Thermostat. However, the Coolant in the Block and Cylinderhead when heated should rise up to the Cylinder head till the temp is the same throughout the coolant system.
https://en.wikipedia.org/wiki/Thermosiphon

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84 300D, 82 Volvo 244Gl Diesel