I saw a post on the now defunct overgrow.com, where a guy had a cabinet about the size of a C-22, and a 1000 watt HPS light. He decided to try the 1000 watt light in the cabinet, and it worked fine once he got his ventilation going good. 1000 watts is too much light for that amount of area, and he knew that, but he already had the light, and wanted to see if it was doable, and it was. So if 1000 watts is doable, obviously 600 watts is too. I don't think it will result in a major yield increase though, as the real benefit will come from the far ends of the area, where the 400 watt light might not be providing a lot of light.

A cheaper and more flexible alternative would be to put 4 30 watt CFLs at each end of the rectangle (where the coverage from the 400 watt light is at its weakest). This would provide 640 watts, but I think you'd get more light at the edges that way, and thus more yield. And you could turn off the CFLs but keep the 400 watt light running if heat became an issue, which is not an option when it's an all-or-nothing one-light situation.