An update to my previous post ^^above^^.
To test if the degradation of performance was caused by upgrading the GPU driver (see the specs), I restored both the C: (Windows OS) & D: (MSFS) partitions from backup version made 2 days ago, so restoring the v32.0.23017.1001 GPU driver (Adrenaline v26.1.1). After updating MSFS to SU5.1 (the backup had the SU5 .27 version), I recreated the same flight from Shepparton Airport (YSHH) to Shepparton (YSHT) in the same aircraft (Flight Design CTSL), at the same altitude ( ~1,000 ft +/-). The same streamed addons were left enabled. Since I did not exactly match my circling of the town, this flight was 1 1/2 minutes longer than the former.
This time, performance was on-par with the SU5 release version. There were no microstutters at all, and only four short stutters that immediately preceded my entry into the photogrammetric area, which coincided with an increase in network download activity from the mid-teens Mbps to 65-85 Mbps +/-. Speculation: is the photogrammetric data stored in the same location / on the same servers as non photogrammetric data? Could these four stutters be related to MSFS “switching” to the different location to get the photogrammetric data?
FPS along the Goulburn river as I headed north from YSHH were in the high 90s to 105, dropping slightly once inside the photogrammetric area down to between the high 80s to mid 90’s, which is what I’d expect for this type of scenery. The only time there was a short (about 10 seconds) drop into the high seventies was when overflying the suburb of Mooroopna, just to the west of the main town.
So, IMO, unless future flight contradict the above results, the poor performance noticed in my first flight in this beta build was caused by the AMD v32.0.31007.1017 driver included in Adrenaline v26.5.1, and not this MSFS SU5 .28 build.