yet another scupper question

[oldfielder]

Ok-So with my stern heavy i/o I've pretty much ascertained that going out the transom will require my scuppers to be below the water line or right at it. I think this pretty much rules out the ping pong balls.
So here's the question: If I have scuppers below the line will they drain fine if I just use thru-hulls with flaps like the gemlux. I'm happy with the floor drains for most applications, but every once in a while I have the nightmare vision of taking one over the bow in montauk and needing that h2o out of there fast. I've seen all kinds of thru-hulls below the waterline and I assume some of them are drains.
I know this also adds another element of gate valve or seacock into the equation-I'm really just wondering about the drainage factor.