A 2 stage unit provides little payback. But does increase comfort greatly.
If you look only at performance numbers, you may inadvertantly pass up on comfort.
With 2 stage units, many peple find they can set their stat 2 or more degrees higher in the summer, and feel cooler then they did with their single stage. This often saves more money then the operating cost calculators show.
I haven't had a customer yet that had buyers remorse because they got a 2 stage system. I've had more then one, wonder if they shouldn't have gotten the 2 stage instead of the single stage they got.
Usually after they hear from or visit a friend that has a 2 stage.