Put it to you this way...it's not that you will automatically go from, say, .5gpw to 1.5gpw if you go from horizontal to vertical with all other factors remaining the same. What vertical will afford you is a higher margin of error in terms of yield, and it will make it easier to get a higher yield with the same container size, media, etc. simply by virtue of the efficiency of the system and the amount of light that actually hits the plants when compared to a horizontal setup.
Not sure if I'm explaining that well or not, I feel like that doesn't make sense as I write it even though it makes perfect sense in my head.