Exhaust and Carbon Filter Question

I have a 7x7x8 room with 2 600w lights in a cool tube. The room has a vent from the central air in it. I have a 720 cfm fan for the cool tube. I also have a 720 cfm fan for the charcoal filter I have that is rated at 800cfm. What is the best way to run these fans and charcoal filter. I can cut holes in the ceiling and use a window that is in the room. My concern is that if I move the air out of a 400 cf room with a 720cfm fan I will be exhausting and wasting my AC. Should I go get a smaller fan and carbon filter? My main concern here is using the smallest amount of power I can.

I've read lots of posts on exhaust, but no one really talks about losing your cool air to exhaust.

How small of a fan can I use to cool 2 600w hps?
Should I get a small 150cfm fan to exhaust the room and run the charcoal filter as a scrubber with no exhaust?

Help please.

I would run a duct from outside to the cooltube and out of the room so the lights dont rob your cool air
next I would look into CO2 and seal the room.-as long as you are pumping CO2,you wont need fresh air.
if CO2 is not possible,either go for a smaller fan or find a timer & run your fan for a minute every 1/2 hr or so to move fresh air in. a small oscolating fan in the room will help blow the air around & keep stale air from collecting on the plants

if i got a timer for the fan, is the charcoal filter still going to work?