R407c is basically R410a with R134a added to make the pt curve fall (lower) into the range of R22. For HVAC applications, power and capacity are very close to that of R22. The only drawback is that R407c exhibits glide, so it doesn't quite condense and evaporate at constant temperature. It has a bubble point and a dew point, between which the latent heat transfer is not exactly constant. This translates to subtle differences inside the working system, like increased pressure drop and mean temperature difference between condenser and evaporator. In retrofits, minor adjustments in the metering device can balance the system to provide capacity or COP requirements. With a system designed around R407c, everything can be designed and spec'd so the system performs very much the same as an R22 -based system.
Ultimately, with any heat pump system, you are faced with a temperature split vs efficiency trade-off. The higher your indoor supply water temperature needs to reach to meet heating demand, the more of an efficiency penalty you will pay to reach that temperature. I believe Dupont has now released R407f, which is pretty close to the same thing as R407c, with the blend optimized for a little better performance at high delta-t operation.
Oh, and it can't be just dropped into an R22 system; it requires synthetic oil. Along with the other general quirks that the other 400-series refrigerants share. Super clean on the inside, zero tolerance for moisture, etc.
This topic goes right along the lines of
this thread. Doing accurate heat load assessments and designing for appropriate heating requirements is not fun or exciting to many, but it is very rewarding in the end.