Depends on the spread. If the light is higher, and with a wide reflector, then the light won't be concentrate enough. If it is a small tank, and the light is close to the water with good reflector, then I feel 150 W is more than enough.
Hmmm... well I was thinking of a 48X24" footprint that is 12 or 13 deep. I guess I'm locked into a couple of 250w. I even have an extra 400w laying around, but I think that would be too much and would prolly bleach everything out.
I think on a spread that large you could keep it low and have sps in the center of the tank and softies on the outside (horizontally speaking) I had a 150 over my 40 gall but it got too hot (the water)
i have a single 250 watt pendant over a 40 breeder, and all the sps does very well with it. i have it about 20" above the water. it's in one of those sunlight supply pendants.