It is not clear why your heat pump cuts over at 40 degrees if it still has COP of 2 at 8 degrees. Presumably a cost-effectiveness calculation if gas is cheap in your area. If you had heat strips instead of gas, the "cut over" temperature would be much lower. If your unit is capable of operating at full capacity below 8F, it would likely only use resistive heating below 5F. For most climate zones, that is fine and your unit does not sound like one specifically deemed a "cold climate" heat pump, the which now regularly achieve full rated output down to -15F. Of course, it's only rarely that cold even in "cold climate" zones so operating down at a COP of 1 for these short durations is not a real problem. The impetus for systems designed for lower temperature operation is not really aimed at those with cheap gas but at those living where heat pumps were previously only feasible for heating if using much more expensive ground source systems. Achieving year-round heat with only the heat pump eliminates the need for a furnace so you have to include this missing gear and installation cost in your analysis.