My understanding is about 550 CFM is the max for 4 inch pipe. You can over pipe and lose velocity to where the big chips will not move through the larger pipe and cause clogs.
You are correct it is airflow not horse power. But it is not just CFM but also FPM. It is a combination of CFM and FPM which creates a good DC.
My other understanding is you are better on the high side, maximum through put, of a cyclone than on the low side. You get better fine dust removal on the high side of a cyclone because the fine dust is thrown harder against the side of the cyclone to fall down rather than pass through.
This is all my understanding which could be completely wrong because I am no expert. It is just what I have read the last month.