From what I have seen:
Low flow - slower growth, some corals only extend their polyps at night or after feeding time. I also see thiner branches, and more spindly looking growth. The corals are also more delicate and easier to accidentally break.
Med-high flow - influences HOW the coral grows, which directions, and how thick the brances are.
High flow - you can literally blow the flesh off the coral if its too high. Although if its not too bad, I have seen more rapid growth, larger encrusted bases, and different colors in some species.
I personally don't see a big correlation between polyp extension in high flow and lower flow areas of my tank, but I have seen it in others' tanks.
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