#HeatPump update, now that it's been up and running for several months, and now that we've had a frosty day down near freezing.
I set the thermostat to 19°C in August, and it's had no problems keeping the house at around 19° ever since.
As expected, electricity use has increased as the days have gotten cooler, and efficiency has decreased a bit as the air has gotten colder, but it's still chugging away at like 300% efficiency.