There is a great (German) database with heat pump performance. You can switch to English on the right top: https://www.waermepumpen-verbrauchsdatenbank.de/index.php?bu...

There's quite some monthly data on Air/Water systems (extracting heat from outside air, into a central heating system). I suppose good Air/Air systems could be similar. Indeed above >0, or at least >5C it would be beneficial (even more so if you also have PV installed)

Depends, some do a lot better than others, but in general until it is below freezing by a bit heat pumps are overall more efficient.