Hi Erik,
Not sure I agree 100% (but kind of do), if you look at the metadata of a photo the gopro writes it tells you what the exposure time is (note a video doesnt have this metadata). With an ND8PL filter in moderately bright light I was at 1/60 (so close to the 180 degree rule). I suggest those using filters have a look at the photos metadata to work out what your shutter speed will be. Of course this only works if you take a pic just before you record your footage.
It was just annoying you had to take the pic, then your footage and then review afterwards to get the shutter, this update eliminates that part. I only did this when I was really interested.
I'm curious about what you are seeing (that you require even higher level filtering), I'm hesitant to update my GP firmware as I kinda like the turn on solo and my camera just starts. I have no issues with updating firmware its just that I upgraded from a GP3+ Black to the GP4B for this reason, so I'm likely to wait till that is fixed (or if there is a long wait I may just do it anyway).
For me an ND4 filter on a semi bright day results in a 1/273 shutter speed, a bright day on an ND8PL will be about a 1/80, so yes I agree that most using filters were unlikely to be getting close to the 180 degree rule on a bright and sunny day, but overcast and using an ND8PL they might be getting close. I don't prescribe to it having to be exact, I think if its roughly right then it turns out good.
Chris