Modifying a dual plane??

Cutting down the divider is essentially turning it into a single-plane; might help you if you are looking for an edge at the races and can't swap to a proper manifold but on the street it won't do anything for you. Might even take some torque off the bottom end.

EDIT: If you really want it would be a better idea to experiment with an open spacer under the carb that will have basically the same effect