Pinto, I don't believe that you have any idea of what we are talking about.
The depth of water in the hole never gets any deeper than 300metres (or whatever the required amount between recirculation cycles is)
The generating head is from the top of the storage at the surface, to the generator, period. It only varies by the variation in the stored head.
Once the water exits down the tail race, it is irrelevant to generating capacity or head. The depth in the hole is only relevant to the energy required to pump it back up to storage at the surface.
