It's a Carrier 38MARB Performance Heat Pump, and it SAYS it's rated at 100% efficiency down to 5 deg F but I can tell you from experience that it starts buckling below freezing. I have monitored it over a whole winter season and when we hit the 20s it starts running defrost cycles every 15-20 minutes. Each cycle takes about 5-10 minutes to defrost. It's not able to keep the room warm.
Granted, this is a 500 sq ft room with a wide opening to the next room and a 25' ceiling. I don't think it was a great use-case for heating in general. For cooling, this thing is irreplaceable. It barely breaks a sweat and uses almost no energy, while keeping the room at any low temperature I set it to. I've yet to see it have any issues cooling, even when it's 90F and humid outside.
That is very good to know about the high heat model. Thank you.