I'm glad I stumbled across this. The improvement in efficiency you've realized is most impressive. I keep kicking myself for the way I bought the central unit in my house. I have a 12 SEER AC unit combined with a gas pack for heat. The gas pack is bottom of the line 77% efficient.
It's somewhere on my to-do list to replace and I keep thinking how a geothermal unit would be great. {Except for that big pricetag
} Whatever I do when the time comes, you can bet I'll pore over every thread I can find here on the topic.