Its called balanced heat exchange, lets say you have one 1k lamp and you put a 360 fan on it and another 1k lamp that you put an 800 fan on, both will go through the motions of drawing heat from the lamps but the lamp with the 360 on it will be 6-10 degrees cooler, this is because the air would have hung around inside the lamp long enough to absorb lots more heat, this is why its generaly a bad idea to daisy chain lamps as the air entering the second lamp is already holding nearly as much heat as its capable of. bit simplifyed but hey...listn to u...OP just asked if he should get a 800-900cfm fan..ur reply is pretty much No thats crazy, thats like 100mile an hour winds..with no heat removal...ahh but go with the 680cfm..dude how can his extra 200cfm hurt..F*** sakes