just for comparison i will tell you what happened to temps in my space when i went from a nonaircooled hood to a 80cfm cooltube then to a 465cfm cooltube. my space was 4ft wide by 2ft deep by about 6.5ft tall. i had a 400hps non aircooled hood, for ventillation i had 2 180cfm exhaust fans at the top and at the bottom a 10inch vent hole with a 10 inch box fan sucking air in. temps were between 79-83 with lights on and i could get the bulb to about 16inches above the canopy. i changed the nonaircooled hood to a cooltube with a 4inch, 80cfm duct fan on it and all the same ventillation in the room. the temps never went over 80 and i could keep the bulb 12inches above the canopy. next i changed the 4inch duct fan on the cooltube to a 465cfm squirrell cage fan and also i got rid of 1 of the 180cfm exhaust fans. once i did this final upgrade the temps were always the same as the ambient temps in my home and i assume i could move the bulb even closer but i need it at least 12inches above the canopy so the light can reach both ends of the closet.
that might help you out a bit to get an idea of which ventillation method would work best for you. good luck.