It's pretty much the same deal with fish tanks. I've seen some amazing aquariums that were very tall, but had a fairly small footprint and according to the 'book' you aren't allowed to stack the fish any more than you would be allowed to stack the chickens when determining square footage. x birds per square foot regardless. I believe personally that the only time elevation plays a role is when / if you have the luxury of having a second floor... eg additional square footage. My house is a tri-level, big enough to squeeze in 6 adults easily. However the footprint wouldn't support more than 3 without that extra 'floor space'. If your coop is 8 feet tall, but only has 1 floor, it might as well only be 3 feet tall... jmho..
Regards,
Mark
Edited to add -
the more I think about it, the more elevation really would play a role. I mean seriously, if you have a 2ft by 2ft 'chicken tower' that went up 30 feet in the air, you could certainly house more than 1 bird in there, right? but you would most certainly need an actual floor for each bird. (as I imagine a yuppie chicken condo in the city with elevators and junk) So I think verticle space is only a factor when you have multiple floors. Roosts kinda count, but not really as that goes back to the fish tank analogy. The square footage 'rule' per bird is the only safe way to make sure you aren't over crowding in my opinion but certainly height does factor in... in a small way.