I recently replaced the air cooled condenser on my heat pump with a *big* bphx. It used to condense at abut 55-60C and now condenses at ~25C with ~1.5K subcooling. I had already replaced the fixed orifice with an EEV, so I don't know what the drop in pressure would have done to the original metering device. I can say that the capacity of the system has increased significantly.

I suspect as long as your metering device can cope with the reduced pressure differential, the lower you can condense, the better. My ground water loop is at about 20C so I can't really get it much lower than I am right now.

I'm waiting on a flow meter to arrive so I can properly quantify the heat being rejected into the water.

Overall, and taking into account replacing a 30W condenser fan with a 150W water pump, my net power consumption has dropped over 750W, but I'm also moving a *lot* more heat. My supply air temp has dropped from 17C to 13C on average.