You don't need to run your MH
for any longer than 6 hours a day. Keep in mind that the summer months are going to be harder on the heat retention than the winter months (try running your heat at 76 while the MH
Raising the 400w MH
up higher over the tank is definitely going to help with the light spread and keep the dark areas under control. By adding a PC
fixture into the canopy you can mask the effects. The canopy should help contain any overspray of the light.
You'll probably want to start your corals low in your system and move them up over the course of weeks as they become acclimated to the stronger lights (new ones as well unless they were kept high under 400s originally). Some LPS
and Monti species will want to stay on the bottom as they don't really like a lot of light to begin with.
Coralvu makes a ReeFlux bulb for the 400w MH
in a 10K that is supposed to be out of this world for color and growth. I run 250s over my 120, but know others who've run 400s over a 30" tank with no ill effects.