Anecdotally, I have a hard time buying the stat that homes today are only 31% more efficient than they were in 1970.

I lived in a fairly large, brand new home for a couple years, and my energy bills were way lower than my friends who lived in smaller, older homes. Some of this was a much better insulated and tighter sealed house, some of it was more efficient HVAC units. IIRC, the air conditioner installed with the house had roughly twice the SEER rating of my parents and parent-in-laws units from the late 90s. And this was in a fairly mild northern climate - you can get units with even higher ratings, and if you're in a locale such as Arizona, Nevada, Texas, etc, I think you'd be more or less priced into doing so.