400 mh

The situation you described is exactly why people used to commonly use 400MHs. A good, modern reflector, such as a lumen bright, or lumenmax, however, will give you more PAR out of a 250MH, than out of a 400 with those spider reflectors. Also, the XM15K is about the least efficient bulb imaginable. A Phoenix 14K will give you almost twice the output. The situation you describe, even with 400s, may not get a lot of light into the tank, however, a better tuned setup, with 250s will.

Depending on the corals, even with good reflectors, and an efficient bulb, there could be a benefit from a 400, as certain corals may color up better and you will have more light at the bottom of the tank.

 

Depending on your stock, depth of tank, and color temp of the bulb you run. For myself, I ran a 2x400W over my 110G, but it's a Sfiligoi fixture with 2 reefbrites. Basically two 400W bulbs side to side.



20K bulbs like an XM which has lower par, have switched to a 250 Radium and 400 XM 20K, but never noticed a difference in heat. My tank doesn't evaporate too badly, but I have a 1/3 hp chiller, which rarely runs, except on extremely hot days.