You have it right 400+ cfm per minute should cure the problem. I have 800w in a 5x5x8 and Im about 5 degrees warmer than the outside air. I would reccomend a sunleaves windtunnel. They are pretty quiet for their power. Plus FDD pointed out they have a polymer casing, which means that they are insulated from sound unlike metal housings. I went through 4 fans and if I would have just bought the sunleaves windtunnel first it would have saved me from doing 3x the work and spending twice the money. Just do it right the first time.