Just to clear this up a bit, your boat itself is NOT "self bailing"!
The cockpit (the area where you can walk around) of your boat my be
"self bailing" through the one-way scuppers in the transom as long as
they dont get clogged or when water runs out over the cut-out in the
transom. Since you have a bildge with a bildge pump, your bildge is
only "self bailing" for as long as the pump and float switch operates
correectly. I would consider this as part of the "self bailing" feature
of your boat.

You should NOT be counting on water rushing out over the cut-out in the
transom! Why do you have that much water in this boat? Why are you
backing down so hard that the water is coming in the boat over the
transom? What make boat is this and what size engine is on it? Where
are you running this boat?

I would not consider blocking the cutout in the transom, not a good
idea.