Ok back up:
When was your house built?
What kind of insulation do you have in the walls?
What kind of insulation in the ceiling?
What Climate zone do you live in?
I currently have a hybrid system, took me a while to get it right. I do not like my heat pump at all. It might be too strong a statement to say it was a mistake, but there is a massive difference if you live somewhere that actually get cold.
Heat pumps are more expensive then gas, I have had discussions about which is better for the environment and I think that the HP is better, however its way more expensive then using natural gas.
Your house is also freaking huge, does it not already have multiple furnaces? If you had second furnaces for each floor a second floor HP might be a decent call. The best part of the HP upstairs is in my 100 year old house the upstairs used to roast in the summer due to design and thermodynamics. Having the HP upstairs lets it absolutely crush the hot air.
In the winter the reserve happens which is nice. The gas furnace heats the first floor and some of the second, we keep the HP about 2-4 degrees cooler than the gas furnace. As a result it has much less work to do (read much less cost)
I think HP are fantastic if you live below the Mason-Dixon line, I think they are marginal in climate zone 5, and not worth it in 6 or 7. Depending on your location, if cost savings is your primary concern. I would highly doubt HP wins they cost more to install, and more to run versus gas. If Environment is your main concern HP likely wins. But yes there is a bunch of noise out there saying that HPs are going to save the world, they have their place but are not a magic bullet. Most houses would highly benefit form additional air sealing and insulation before addressing a expensive piece of equipment.
