I ended up doing some very basic wireless latency testing from laptop to router and also phone to router, to compare the older mwlwifi driver to the newer one.
I was actually expecting some differences, but it was basically identical. Not higher or lower. This was using 5.15 kernel.
It’s so strange how different users setups can produce such different results. I’m actually quite pleased with mwlwifi over the past year or two. The first bunch of years were a disaster though, for sure.