Keeping 5x over the entire winter includes some pretty cold nights. And ground source systems can improve even this by quite a bit.

Here we go: https://industrialheatpumps.nl/en/how_it_works/cop_heat_pump...

or better this pic: https://www.researchgate.net/figure/Performance-map-of-the-h...

> The heat pump model is based on linear interpolation in a performance map retrieved from manufacturer data (Daikin Europe N.V., 2006).

And that model is half as good as the modern one I linked:

> The nominal coefficient of performance (COP) is 3.17 at 2/35ºC and 2.44 at 2/45ºC test conditions (i.e. air/water temperature) for full load operation.

And even then it gets a COP of 2 at -15C. So modern systems getting to at least 4 at -15C seems likely. This claims there are low ambient mini splits that maintain 100% efficiency down to -15C:

https://www.ecomfort.com/stories/1341-Keeping-Your-Mini-Spli...

That should cover most places but for really cold climates ground source heat pumps seem ideal. From what I've seen there are simple solutions where a single vertical hole is drilled with common well drilling machines and a single tube that has the loop inside is driven down. Makes it easier to retrofit and implement in small properties.