Hi Ozzy,
I've never seen any meaningful info on the purpose(s) of the firmware upgrade, and I too didn't perceive any sonic differences when I did the upgrade. Just guessing, but perhaps one purpose had to do with optimizing compatibility with the most recent version of the calibration software, which in turn might have been updated to optimize compatibility with Windows 10, among other possible reasons.
There was also a separate upgrade to the display processor firmware that is used by my HDP-5 and the PreMate+ to control the touchscreen that is provided on those models. That upgrade provided some enhancements to the touchscreen and I believe also enabled the ethernet interface and "Roon readiness" that are provided on those models. (I don't use those features, btw). But those things aren't relevant to your PreMate.
Best regards,
-- Al