Try ourcoolhouse.com They have live streaming data and trends for a geothermal heating/cooling system.
I researched a Geo system with an IC engine as the prime mover, but it had some definite shortcomings. A Stirling is definitely the way to go. Good luck.
Heat Pump COP and perpetual motion machine
I'm a little fuzzy on the efficiency calculations. Wouldn't a heat pump with a COP of 5 put out five times the energy it requires to run? In that case it could be coupled to a Sterling engine with, say 25% efficiency for a net efficiency of 125%...