Not quite. I've looked through the Goodman specsheets, easy to get and plenty of detail. Most of their heat pump systems have slightly lower COP at their lower stage. The savings seems to be during cooling and of course 2 stage units are designed more for dehumidification and comfort anyway and it happens to be that they size a larger coil on the more expensive two stage units too for added efficiency. Since the heat output is low they might as well run high stage and shut off, then again when they are actually running they will need to be running for a long time anyway if they are sized properly for cooling in the north and the unit actually needs to run.
If you have a reasonable gas rate and a well insulated house, the running costs of either heating method is negligible since the heat won't be running much at all above freezing temperatures anyway. It's when you get close to zero and drop lower that you really need the heat, just when the heat pump doesn't have much chance since its defrosting constantly. Try factoring the extra cost of the heat pump into the running cost of a 20 year estimated lifetime of the unit, hopefully it will last longer than that.
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