hmm.. never installed a HD fan man. But, since warm air rises to the top by natural convection, I'd say you should probably direct the fan air flow keeping that in mind....
Then again, it depends on your case characteristics too........ It'll be nice if you could mod your case and install a nice grill infront and install a fan to blow air out. Bah. Cooling is a science in itself..
So you are saying the fan should go directly below the HDD taking up an extra bay? Would there be enough room for sufficient air flow? And what about the component just on the other side of the HDD. Wouldn't that get warm air then? Never installed one, so just curious
What kind of improvements did you observe man? Wouldn't the device above it get warm air then? I always "thought" it would be a good idea to blow air from the side towards the front/back of the case instead of up and down.
EDIT: nm. Didn't see your edit re: the warm airflow.
If you have one you'll understand, theres basicly only one place where the one I've got can go unless you decide to move the HDD bays - but dought it .