Growth directly corresponds with size. A 1" frag can take FOREVER to put on another inch, while a 6" colony can do it in ~2 months. However, clearly your parameters are the limiting factors here. In general though, SPS are pretty fast growers. That is their defense mechanism, grow fast to shade out other corals. (LPS grow slow but typically have sweepers and pack a much more potent sting than SPS). Typically, montipora capricornus and seriatopora hystrix grow fast. Just about any Acropora called a staghorn are considered to be the fastest out of the genus acropora, with some saying that acropora micropthalma (scripps staghorn acropora) is the fastest. How is the flow to the acropora that are growing slow?