I've got a 24"x42"x96" closet grow with a 600w HPS running with a 4" inline vortex fan and carbon filter... My reflector is not air cooled, yet the temps are optimal.
It's funny cuz the fan I got is around 160-180CFM and the closet only measures 56 cubic feet, I got the fan running on a timer for 10 minutes on then 20 minutes off, all day/everyday... And the room that the closet is in is about 512 cubic feet, and it's the room where I keep the litterbox for my two fat cats who shit a shitload. The filter/fan combo not only keeps the bud smell in check, but it recycles the air in the entire room and also suppresses the litter box stink.
That's how well these things work.
So is it overkill? Maybe a little.
Can you get away with a passive intake? Definitely, but if you are not going to have the fan running at all times, then you should think about covering up that intake with one of those cheap carbon prefilters that you use for air purifiers. They cost around $10-20 at walmart and will prevent any odour from escaping your intake vent while the exhaust is off.
