Yes, they are Heat Pump thermostats equipped with Em. Heat mode.

They are Honeywell Vision Touchscreen model RTH8500D.

They are also designed to work with other heating/cooling systems as well. The type of system is configured in the installer setup which I have confirmed is properly set for a Heat Pump.

Can anyone tell me what would be the effects of overcharging the freon in a system? Would it ever produce a level of inefficiency that would cause the AUX HEAT to constantly supplement the heat pump?

The more I think about it, the more I realize that the heat pump compressors were running a lot. From the electric bill, the AUX Heat must have been running a lot too. Yet, we were never too hot and the temp never went above the setpoint. It would seem to me that if the heat pumps were producing sufficient heat and the AUX Heat came on as well, it would shorten the cycle causing the heat pump compressor to run much less overall. I recall everything running quite a bit leading me to believe that perhaps the heat pump wasn't producing enough heat so the AUX Heat legitimately had to step in to keep the temp up.

How likely is that scenario?

And, if that's the case, would the overcharged compressors be likely to blame, or do I need to look for a different problem?