A COP of 5 was always possible, it just depended on the temp spread and the load. Those systems in the past needed higher discharge temps because the buildings were crap and a bit larger evap made the unit too expensive. Besides, American systems were just AC units with a reversing valve and a very crude timed defrost, never really designed for heating. The real SYSTEM COP includes the pumping or fan power which until recently was a PSC motor and poor bearings. We know better now.
There are thermodynamic limits to the refrig cycle and a COP of 6-7 is probably about as good as we will see for a long while. The COPs of 8-10+ have usually been debunked. That said, the pace of improvements has never been greater, most of which I put down to measurement and controls.
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