What about this???

[Snookerd]

WWP2-I'm with Tarpun, you would have a vacuum with the scuppers at speed, pulling the water away. My SF23 was originally a diesel jackshafted to an outdrive. It has been converted to twin 225's on a bracket. At 900 lbs of motors, the boat still has water shedding ability with all that weight. On my transom I have exhaust baffels that have been converted to giant scupper drains. Is it possible that your drains have a raw water pick up connected from when you had the I/O???