I've found that acanthastreas are very fast growing coral if provided sufficient light. Mine have grown very well about 17-inches below my 250-watt MHs...though not directly under them. I also sometimes feed them chopped mysis, though I don't believe I do that often enough to really enhance growth.
I have two echinophyllias and they, too, grow fairly quickly. But, I've found that they are very prone to adventitious micro-algae attacks, so make sure your water quality is very good.