The main point of dual exhaust is to increase the cross sectional area of the exhaust flow. So you still have the cross sectional area for flow but the transverse muffler will restrict the flow somewhat. If you want to cut down on the noise you have to accept the flow restrictions of mufflers. Otherwise just run open headers. Cars don’t necessarily run better with less exhaust restriction. Depends how the motor is set up. The x-pipe manufacturers swear that improves performance.