I also noticed my tank seemed more stable overall when I reduced my photoperiod from 10 hours to 6 hours. I reduced the photoperiod slowly over about one month. My temps became more stable, SPS growth did not decrease.
Regarding the nearly 12 hour photoperiod in equitorial regions, this does not mean corals are exposed to 12 hours of direct sunlight every day all year around. Depeding on the location on the reef crest and taking into consideration the constant battle between corals to overgrow each other, most corals will not recieve a fulll blast of sunlight 12 hours a day.
There are also a few studies involving zooxanthalle energy production. There is only so much light they can feasibly absorb, and if you have 400 watt halides on above them, it is no where near 12 hours