It effects it in a good way. Lower level equals less suction head pressure on the pump, equals less flow, which means the level in the overflow drops because the gate valve is open too much for the lesser flow. The gate valve is not static, as the level rises in the overflow, more head pressure on the opening, which flows more water. So there is an equalizing factor in play. As the overflow begins to rise, more head pressure flows more water for the given valve opening. Sort of self regulating to a point. Now I only like my water falling no more than an inch, so water will back up and hit the emergency in one inch. If you kept the overflow say 4 inches low...(more noise) then it would have a much wider range of regulation.