That is why I start with the idea of 10x, but discounting for exaggerated claims, and slowing down as the filter media gets full, I probably have closer to 5x. I just never went to the bother of measuring it.
I have seen a post about someone trying that with the Aquaclear brand of power head. They got between 50-75% of the claimed flow. Then they altered the PH and got much better flow.