I was able to get Matlab to chill about the GPU driver after removing and reinstalling with their mpm installer (it was time for r2024b anyway). Games seem fine now, framerates appear normal.
Ultimately I believe my performance issue was due to reverting the cpupower governor in an update. I am able to fix this by editing the governor setting in /etc/default/cpupower thusly:
# Define CPUs governor
# valid governors: ondemand, performance, powersave, conservative, userspace.
governor='performance'
[...]
Followed by enabling and/or restarting cpupower.service.
This doesn’t appear to persist through a reboot, but this is addressed further in this thread:
Via paclog I can see that power-profiles-daemon was upgraded (0.21-1 > 0.23-1) on my system 10/01/2024, so I expect this was probably the primary issue. I’ll likely remove or disable that daemon out of convenience. Discovering the issue with the Navi31 GPUs is just a bonus, those threads suggest that the TEMP_HOTSPOT error is “cosmetic”. As for the GPU confusing Matlab, I expect this is just because installing Matlab on Manjaro is tentative at best and these things are to be expected – I suppose I’m done here!