I suggest another test , first thing on a cold morning, start the heater and see how much it warms up,the interior in 15 minutes, and then see how long to make a 20 degree temp difference.
Also it would be good to know how warm the water in the tank is after an hour of cooling.
The questions to answer are, is the constraint the radiator or the water heater. You will also get a better idea of how much real impact it will have on a cold night. ( also keep in mind that a couple of warm bodies will also add some heating). Personally I think it is probably fine as is, but it would be nice if it didn't have to run all night.
Tom