I have 2x viparspectra 165w on my 75 gallon reef at 12" above the water. I use glass lids to keep the fish from jumping, but have blues set to 100% and whites at 15%. Blues from 10 AM to 10 PM and whites from 12 PM to 8 PM.
My GSP grows on a shaded island at the very bottom of the tank, shaded by both rockwork and the cross brace of the tank. It thrives on the very bottom, doesn't do as well higher up. The top of the tank is pretty much reserved for Acropora, the RBTA, and Blue ridge coral, which all thrive for me under ultra bright lighting. It's definitely too bright for some stuff toward the top. I tried the big sour apple birdsnest colony and nuked it. It completely RTNd within 48 hrs.