I'm running a 600W hps in a 2x4x5 tent. I cant close it without the heat skyrocketing but the extra light is providing the necessary forgiveness and I got two babies growing nicely. Just lowered it a few inches, actually.
The only thing seems to be like the humidity gets zapped out of the air so sometime I put a bowl of water under the light to compensate. Works like a charm.
*Edit: previously put 2x2x4...no idea why hahahha