Hi Awince, when we talk about water flow, this does not mean turn over through the sump, if you turn your entire water volume through the sump 2-3x PH that will be fine. The rest of the flow comes through pumps in the display itself.
First off lets not confuse softs with mushrooms. Mushrooms require a fairly gentle flow of between 10- 15 x ph.
There are certain LPS that require more flow than others, but generally the 10 - 25 x ph.
The SPS and softies actually enjoy the high water flow 20 x +. The reason folk say that softies for lower flow is that they can tolerate the lower flow but certainly thrive in the slightly higer flow areas.