For it to work, Apple would need to make it known (spec sheet) AND, it would have to actually matter for most people. Unlike RAM size where 8GB vs 16GB can be "felt", virtually no one can tell the difference between 3000 MB/2700 MB and 3500 MB/4000 MB sequential SSD speeds in everyday tasks.
Hell, most people would never know if their SSD was running at 500 MB/500 MB or 3000 MB/2700 MB.
I'd much rather Apple cut costs on the SSD sequential speed over increase the base price of the Mac or cutting elsewhere.