Hmm, that's interesting, we have fairly large temperature swings (typically -15 C in the winter to 40 C in the summer), so it'd be interesting to see if the heat pump could replace the AC unit and the gas heating.
I'm in a similar zone and replaced (delivered) propane with a ground source heat pump 2 years ago. Constant temperature indoors (3C warmer in summer than winter) with plenty of AC capacity to spare. Breakeven is about 5 or 6 years.
They work okay in my area for -1C to 46C, imagine for your range maybe you just get a model that has improved heating over a basic one, whether that means ground source, more stages, or a heating element.
