I worked for a chair that studied this kind of stuff - your best bet is to figure out how to reduce the temperature lift. Due to the way the Carnot equation works (at least, that's what I _think_ it's called, this is all a while ago), that really get's the heatpump efficiency singing.

reduce the heating temperature by increasing the area you're radiating from - floor heating / ceiling heating is way more efficient than radiators since you can deliver the same amount of heat at much lower temperatures to the space.