I Spent a few years working in a 15 ft ally dingy in open waters off Albany. Never did I get a greenie over the back and I did get out in some pretty rough water. The only time we got pooped was when coming ashore in the shallows.
Most of the open style transom/large scupper drain sailboats that I have seen and been on have those drains because they take allot of water over the bow and they have no cockpit combings. Gallons of water flow into the cockpit punching through waves.
I do understand that a larger scupper will get water away quicker and if your sailing ground involves crossing bars and shoals putting in a bigger scupper may be a pretty handy mod, but so far in the open waters I have sailed in, my boat has always remained quite dry in the cockpit other than a bit off spray.