i run a 400w hps in a closet with almost identical dimensions without problems.
however, the closet is in a spare room that i can keep dark. this allows me to leave the closet door open a bit to vent the light. my only environmental controls are a fan on a stick (upright fans? whatever it's called) blowing on the plants and a table fan blowing on the light. as long as you have a way to control the heat i find a 400w to be optimal for a closet that size. anything bigger and the heat would take over, any smaller and yield goes down.
I also run a 400W in a small space. The one advantage to that is the concentration of light is intense. I grow about 2 plants, sometimes 4, but when I grow just 2 the light is so intense the plants love it. The buds get so fat. Even the buds on the lower branches get pretty fat. I love it. But keeping the temp around 79 is a constant battle.
Here is a video of my White Widow grow from last year. I got 2 going in that space. Some might ask why didn't I top or LST or FIM. Well this was a experiment. I wanted to see how it would grow without topping it and how much I would yield. I was very surprised by the results. It grew so tall that I couldn't put my light up any higher. They got to about 4 feet tall. I have video's of them in the earlier flowering stage too.
Here check it out.
[video=youtube_share;iDXnxYhje6M]http://youtu.be/iDXnxYhje6M[/video]
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