Last I read, heat pumps start getting less efficient than resistive heating around -20 C and below, but will still work down to significantly lower temperatures. I don't remember the exact reasons or numbers.
That's exactly what I meant when I said "used to be true". Our HVAC is clearly much more efficient than resistive heating even at -24, at that point it's still blowing a huge quantity of very warm air. Its output is actually much warmer at -24 than at -15, simply because at -15 full power isn't required to maintain 20 C indoor temperature. I'd estimate it replaces at least 2 kW of electric heating and it saves stacks of firewood to boot. Max consumption is 600 W. The manufacturer claims it delivers more heat heat energy than the electricity it consumes at much lower temperatures than that, and it's not enormously expensive either. I think you can get even better models now.
Condensation releases heat, evaporation absorbs heat, and you can change the boiling point of a substance by altering the pressure. The heat pump uses a combined pressure/vacuum pump and a valve to create a partial vacuum in the outside evaporator and high pressure in the inside condenser. That causes the refrigerant to cool to whatever boiling point the pressure dictates, -40 or so, as a part of it boils after having passed through the valve into the low pressure evaporator. The rest of the refrigerant then evaporates while absorbing heat as it flows through it. The refrigerant then passes through the pump into the high pressure zone, where it becomes hot due to the compression. The boiling point is also much higher at the higher pressure, +50 or so, so it becomes liquid again in the condenser while releasing the heat that was absorbed earlier.
The reason for the falling efficiency at lower temperatures is that the pump must perform more work when the temperature differential is larger, as more vacuum and higher pressure is needed to evaporate and condense the refrigerant. When more heat from the pump is lost in the evaporator than the small amount of refrigerant still being moved can transport, you are losing energy compared to resistive heating.
I believe the recent performance increase is due to inverter controlled brushless pump motors, digitally controlled release valves which allow precise control of pressure in both halves of the loop and new, better refrigerant (R410A).
This doesn't mean that Model S' HVAC can do the same, of course, only that it's possible. Also, there are much tighter space constraints in a car.
This is the model we bought.