So, after more testing:
I have both a 16" 5600M based Intel MBP, and a high end 13" Apple Silicon MBP with 16GB of RAM. They are NOT the same when editing FCP video.
For one, as I have stated, most of the old FxPlug plugins don't work. They need to be recompiled.
Second, a nearly 6 flop GPU with dedicated 8GB of HBM definitely beats the M1 GPU. As a practical example, I am able to layer at least 3 (three) 4K videos on top of each other with LUTs and effects applied in real time on the 16" MBP. This is in "best quality" with no rendering; so real timeline performance.
I cannot do that on the M1 machine; it stutters and starts to become unusable. 8K is also very tough for the M1 Mac. It stutters, as does 6K Red Raw and so on. Keep in mind too I am using a basic LUT; If I was using a more complex plugin like FilmConvert this would be even slower. The 16" MBP with the 5600 can keep up.
(To be fair, it can handle two streams - which is impressive; the old 13" machine was useless and this machine costs $1700 vs $4000 for the 16". Also keep in mind this performance is similar to a 5300M based 16" MBP... and the M1 MacBook CPU is faster, though in practical terms you don't really notice for video/gfx work because it relies so much on the GPU.)
An "M2" based MBP will be massively impressive; the M1 Mac barely even gets warm and the battery is at least 2X longer than the 16". But it's not at all a reasonable replacement. Yet.
My M1 Mac is going back.