Yes mad fridgie
can we please have some more data on this machine like compressor data and heat exchangers etc. I think if you changed to R22 for a heat pump gas you would get better results but that gas was good 10 years ago and perhaps not so politically correct today. A sub cooler heat exchanger will increase out put only if the compressor has the capacity for the conditions. You could also experiment with COP measurements with an increase and decrease of the 600 Gms of R410a of 10% and check response with either EEV or some control over superheat at metering device for the changed conditions - either load or ambient.