No matter the light source there is always a trade off with intensity and dispersion. Most people who hate LEDs run them when too low and too high intensity. You are probably spot on about 15". I'm not saying it would be optimal but if someone wanted to save money and put one over a 4 foot tank and raise it up high they could and still get the intensity to grow everything but the most demanding corals.
Look at all the fish store, coral farms, there radions are like 30"+ over there holding tanks. Yes they are very shallow but it drives home my point. If you have enough intensity why not raise it as high as you can for much better spread, blending, and coverage/$.
My favorite thing about raising them up higher is getting light out of my face. I'm short and the tank looks much brighter when I'm not staring into bright diodes