Hmm, I ran mine rather high in the tank when I had MH. The trumpets like a lot of light, especially the krytonite variety. Although not a hammer, I've had my Scuba Steve and Pink Tips high and low under MH with no ill affect to the coral. Flow is an issue though, if too much.

The MH that I used was 400, 250, and 150. I'd almost say that you can't get too much light (recognizing that this is not always the case), but fading of the coral is an obvious sign that you aren't getting enough light. I've seen this on the underside of the coral when light isn't penetrating all the way through the main body, or if it's too far from the light source.