I have done random outdoor grows in the past but nothing ever like this. I have a spacious attic and am building a grow tent this weekend from timber frame and silver colored roll insulation. I will be using a single 300W LED. I will probably fab a 3' x 4' x 60" tent with a vent and a fan...
You should be exhausting the carbon filter outside of the tent to maintain negative pressure. You will also need 'make up air' coming in from outside. Also, lower CFM's (but not too low) means longer dwell time in the activated carbon.
Lower CFM's are better for dwell time in the activated carbon. If your fan is generating too many CFM's the odorous air is passing through the carbon too quickly. Lower CFM's also = less noise.