It's a combination of things.
or are the higher SEER ratings simply from the reduced amp draw of the ECM motors vs. the PSC motors?
It is my understanding that operating a compressor at lower capacity within a given system increases efficiency. Here's why: The compression ratio determines amp draw relative to capacity. Having oversized coils improves heat transfer, thus allowing a system to run lower head temperature and higher suction temperature; this results in a lower head pressure and higher suction pressure.
Your existing furnace control board most likely does not have separate relays and lead connections for two-speed cooling. (Blower speed must be reduced for second stage operation - if not, humidity removal will really suffer and it will be impossible to get the charge right for 1st stage operation) The installer would have to rig it with a relay to switch between 1st and 2nd stage cooling speeds. (Not recommended, blower speed still might be too high on 1st stage)
I've discovered a natural law - everything gets progressively worse. Things only break down and become depleted. Life isn't worth living and everything is going to hell.
Death is the messiah. Everything else is irrelevant and arbitrary.