Just as a comparison. I use a 4X4, 7 foot tall tent. Leaves me about 5.5 feet of headroom.What would venting the heat out really require could I just vent the other end back out of the closet and into the room
With my xtracooltube, and 600watt hps, I don't need air-conditioning. Even during mid summer. The canopy never gets above 30'C for more than a few hours.
When the room temperature is low, the canopy sits around 26-27'C. Which IMHO is just right.
You do want to be recycling your air, in the room though. Not so much for the heat, but more the fresh air.
Just having a couple open windows with a draft would be sufficient.