that 400 hps is kind of risky. i had a closet amost with the same demensions(28''*25''*6') with a 250 hps from htg supply and i had hell keepin the temp down. it was 90+ degrees in there witha circulating fan in there. i had 2 buy a small inline fan with ducting an suck cold air from outside to get the temp to 82 degrees. the area is to small to put a air hood there. if it was summer i wouldnt be able to grow in there, it would be 2 hot. the only thing that saved me was the cold air from outside sucking it in threw the fan. my opion, i would go with the 250hps only because of the heat problem, but if you could work out a system that can work with that 400 hps, go with it.