[slightly scared posting here, but it was the start of this thread that inspired me in the first place so hopefully that's OK, and am also a big fan of SQM which transformed our lockdown life, so anything I can do to contribute...]
Not quite the same tests you wanted @dtaht and not from Head but I have been testing OpenWRT 21.02.1 vs the stock firmware on a Unifi 6 Lite (mt76).
Setup is a linux box running variations on flent rrul_be, wired to an OpenWRT router which in turn is wired to the AP. Ran netserver on an M1 MacBook Air to act as the target, so all LAN for the test.
I also tried some (not very scientific) additional loading of the AP by having a few other devices connected to it streaming videos while the tests were running.
BEFORE - Unifi stock firmware (current at time of writing).
AFTER - OpenWRT 21.02.1
I was amazed by the difference to be honest - so I then spent a happy day or two reflashing all the other APs in the house.
Happy to run others if the test setup is adequate, and/or to make adjustments to tests. Could even reflash one of the APs back to Unifi stock if that's a useful comparison.