You can safely choke back the return pump at the exhaust port. It's not bad for the pump. On the contrary, it will lengthen it's life. That said, I don't really understand the issue. If this is a gravity fed overflow, meaning, the tank is drilled, it's only going to drain what you put in it. No faster or slower. Try sliding a piece of airline tubing down the hole a bit. see if that helps. It should pull air from the tube and calm the toilet flushing sound, and regulate the flow a bit better.