SPS only NEED to use their polyps for gas exchange. More PE at night and less during the day with photosynthesis happening. Some SPS can use their polyps to try and catch some food, but they probably don't catch much in captivity, but this also happens more at night in the wild. LPS and NPS have different levels of food catching than SPS do.
In the same system, frags that I put in my fuge with no fish have maxi PE whereas the ones in the display have little - they all grow the same. Don't sweat this at all. Growth and color is about a million times better indicator than PE.