MH wattage & depth

[andresont]

I think there would not be a huge difference in heat between 250W and 400W.
heat will be an issue regardless. unless your tank is in the basement.
May be you need to decide what kind of SPS you want and how much light do they need .
Even though i am T5 user; in your situation (you already have the fixures) i would get a good air flow through the canopy and go with 400W+T5 AquablueSpecial (60%actinic&40% blue)for getting super bright colors out.
Just make sure to get a good ballast like M80 and good reflectors all these things are KEY points especially reflectors.
If you look at Web based SPS retailers that is what they use 400W+T5. seems to work just fine.