Flashover,
Read the thread below entitled "heat pump for heating water", a poster called dj01 has a thermia heat pump (air to water ). In addition I visited a house with a thermia air to water heat pump & the couple were very satisfied with its operation. I would imagine that the thermia ground source heat pump would be equally efficient. Having said that from my own experience I would recommend that you go back to the supplier of the heat pump, find out if they installed any locally & ask them to arrange contact between you & someone who has their system. People trying to sell you stuff can be eh a bit flexible with figures regarding efficiency etc, you cant beat first hand information from someone who has no real interest in what you spend your money on.