It has been my understanding that the best thing an exhaust system can do for a 4 stroke is get the exhaust gas out if the cylinder and exhaust port as quickly as possible.

Headers help by keeping exhaust from adjacent cylinders away from the exhaust port and tuned headers use the pulse from the other cylinders by pulling the exhaust away.

Crossover or H pipes help in much the same way by balancing pressure.

How then does backpressure help under any circumstance?