I've got a 16kW Samsung ASHP and if anything I think it is oversized. The house heats up within a few hours so it doesn't run it continuously, which I believed was the most efficient way in winter. COP seems pretty good at 2.89 during the latest cold weather inc defrost cycles.
However, previously we had an one room oil rayburn and log burners so have nothing to compare it to.
When installed we did insulate and it was a complete new CH so rads are sized correctly. We also kept the 2No wood burners as supplementary heating - which when working from means I'm not using electricity to heat the office.
