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.

I just plugged the numbers from a Water Furnace geothermal heat pump in to the mix. Factoring in open loop with a shallow well pump and 50°F entering water temp, the numbers are disappointing. Efficiency does beat out everything else... until I factor in the 1.2kW for the well pump running constantly to supply 11GPM of water to the thing. At that point it is more expensive to run than my current 80% gas furnace. In fact, a 4 ton ASHP at 25°F OAT still beats out the rest by a few cents per hour. :(

Looks like it might just be a good idea to eventually replace the central AC with a decent HP and set it up to run only when the outside air is at a decent temp. There's really no major benefit to modifying my current system, other than the need to replace the old AC unit, which we don't use much anyway.

Edit: I didn't account for the cost offset of using the discharge water for things like lawn watering in the warmer months. However, we don't use the AC unless ABSOLUTELY necessary in the summer, and I really wouldn't have the need to water the lawn when it's 45°F outside and the unit is not running much for heating anyway.