or are the higher SEER ratings simply from the reduced amp draw of the ECM motors vs. the PSC motors?
It's a combination of things.

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)