A quick search did not result in any official changelog, but some forum entries (e.g. [1] and [2]) suggest that random read + write performance under windows seems to have been improved.
FWIW: Write speed does not improve on Ubuntu 20.04. Still stuck at 400 to 500 MB/s. This was the issue which made me want to install the new firmware, and there was nothing on the drives which I cared about.
Upgraded from to 3B2QGXA7 to 5B2QGXA7.