Reason I recommended the amount of CFM's I did is because my setup with a 250 hps, it hits 82-83 degrees in a similiar sized setup, with the ballast in the grow space. I replace my air every roughly 10 seconds......thats alot of fresh CO2 flying by, plants moving around constantly, doubel scrubbed...........yeah, you can stick your face in my exhaust and not know what is on the other side.
I was just making sure he didn't cut himself short and have heating/co2 problems. And with a 400watter in that small of a space, it's going to get hot especially if he puts the ballast in there as well.
Can't be too careful when you share walls with your neighbors..................