Yes, GenX pumps are loud. The pipe size makes a big difference. The volume of water is greatly increased as the diameter of the pipe increases. The bulkhead is not the only factor to how much water gets to the pump. How it is plumbed and what kind of drain is attached to the bulkhead affect the gph as well. Usually you want the biggest drain line and then go with smaller return lines to the tank. Reducing the returns at the last possible point also helps reduce back pressure on the pump.

Reducing the outputs from 3/4in. to 1/2in. doesn't reduce the overall flow. You have the same amount of water being returned from the pump either way. Reducing the outlet size simply means you are going to increase the pressure of the water coming out. Kind of like when you put your thumb over the water coming out of a garden hose.