not to sure on the sea of green apart from its to do with taking loads of clones and producing replicas of the mothers, but can say you will need really good ventilation for a 400 watt in that space as im growing in a similar sized space and i started with a 400 watt mh with an extractor unit and fan blowing in and the temp was goin well above 90f and there was no chance of closing the doors,i had to get it exchanged for a 250 watt mh/hps system and is much more settled temperature wise and can close the doors without worrying about heat
first pic(getting frazzled with 400 watt)
second pic(doing well under 250 watt)