"jkirkebo"
Again, a proper heat pump with a modern refridgeant (R410 or CO2 or a few others) will work well down to at least around 5 F...
By "well", I assume you mean at marginally greater efficiency, than the resistance back-up?
I really think there is so much variability of heat pump designs and operating efficiency, we won't know the actual energy savings, and cost increase, over the current heater, till it comes out.
My guess-and it is a wild guess-is that adding a HP cycle, at additional cost of cost about $200-400, could increase "average" heater efficiency by about 50%, the largest part of these savings occurring in the higher "not too cold" temps.
At that low ambient temperature, that the design requires it revert back to resistance, it will have no benefit at all.
So at some (unknown) low cold temp, it will produce no energy savings, or extra range, at all.
And, it will be most beneficial, where it is often cold-but not
very.
I doubt it will ever produce "25 miles" of range, except over a long period of low speed driving, ("LEAF 100 mile range" miles) and on 40+ F days, when it operates more efficiently, than when it is colder.