That would give you exactly 7.5 watts per gallon. Which is a very good area to be in. While talking to Robert Fenner, I asked the question "Can you ever have too much lighting?" His response was "I really don't think there is such a thing as too much lighting, because you can never fully replicate the light that the sun gives off. However, you can burn corals, so directing to high of wattage in a small area could be harmful."
In the end, I'd say anything more than 15-20watts per gallon is unneccessary. Also, when adding 400watts, that could be very stressful to your corals and fish. One trick I've heard to acclimate your corals to the increase in lighting is to layer sheets of screening on top of your tank, and each day remove a sheet. This way you are slowly increasing the amount of light the inhabitants of your tank recieve.
Either way, good deal on the light. And your plan of running the lights seems very effective.
125g~Zoanthid dominated~SPS~LPS~T. crocea~S. haddoni~2xSaddleback Clowns~Hippo, Kole, Achilles Tang~Leopard Wrasse~Flame Angel~Purple Firefish~Gobies~Chromis~3xCleaner Shrimp~2xHarlequin Shrimp~Pistol Shrimp