Just my opinion, based on a bit of logic...
If I put a 40 watt light bulb over a 3oz shot glass that is about 54 wats per gallon but it will not grow or keep coral alive.
Wouldn't that be closer to 850 wpg
Anyway, I disagree with that statement. If you could get a 40 watt bulb small enough to directly light a shot glass (obviously impossible, but I am following your analogy), you could grow anything that could handle that intense of lighting. I think a lot depends on depth of the coral and the bulbs distance from the water. You could have a 400 watt MH
18 inches (because of the heat) that would give less than a 250 watt MH
right up near the water. Also, if you had a 250 watt MH
and your coral was 4 inches from the surface it would get more light than a 400 watt MH
at 18 inches below the surface.