where are you getting this 20 times thing from? it's not going to benefit corals at all to have more turnover than you need. as i said, if you have a skimmer that has a 300gph pump, and reactors that all have small pumps, add them all up and you'll know how much flow you need through the sump.
i used a quiet one 4000 (1000gph) on my last system. the tank was a 150 gallon. i grew corals just fine.