Well, let's look at it this way. I've got an Acropora and a clam next to each other, within 4" of the surface, with a 250W DE 10000K within 6" of the water surface. They are doing fine, and have been for almost two years. It wouldn't matter if the tank under them was a 10 or a 1000, they'd get the same amount of irradiance. Heat would be an issue, but not light intensity.
A general rule of thumb for a metal halide is it will cover a 2'X2' area. The decision on how many lights to use should take that into consideration. Now whether you use a 70W MH or a 1000W MH should hinge on how deep of a tank you have, and how much difference you can accept between the surface irradiance and the bottom irradiance.
With a 3' tank, one metal halide fixture will not cover the surface with equal intensity. I found that out with a 70 and one 250W fixture. Acro's would only grow directly under the bulb, outside of that, only softies did very well.
On a 5' tank I have now, I used 3 lights, with the overlap of the bulbs falling over where the braces are on the tank, so the light intensity is more or less even across the tank. One thing I can say about the 10 gallon with the 175, the light levels on the top and the bottom of the tank are pretty close to the same!
Another consideration is the distance of the light from the surface of the water. Raising the light will help cover a larger area and keep some of the heat off the water, but it will drastically reduce the irradiance going into the water. I've got a 175 10000K over a 20 gallon cube, 15" above the surface, and it is sufficient for sps only at the very surface of the water.
