ive seen sps thrive in a system where the flow is very low (ie clove polyps in it swayed slowly back and forth) The trick is to make sure debris doesn't settle on the colony. The sps that are in the tank where encrusting montis, pocillipora, stylophora, and a tricolor acro. I myself have a birdsnest in very low flow and it has developed a wide open branching pattern. A frag of the same colony has developed a very dense growth in high flow. The one in high flow has grown faster though, probably because of better nutrient and gas exchange