I've never seen a CoP curve that was not downward sloping with decreasing outside temperature. Do a google image search for "heat pump cop curve" and glance at

I've never seen a CoP curve that was not downward sloping with decreasing outside temperature. Do a google image search for "heat pump cop curve" and glance at the results. All of them are significantly downward sloping because the underlying thermodynamic cycle effect is very strong (agree not the only) limit.