There will always be back pressure, there is no way an engine can exhale gas without some resistance. The ideal exhaust header is one with as little back pressure as possible.

The exhaust manifold is very important for two reasons, it allows the exhaust gases from other pistons to create a low pressure area which pulls air from the intake valves which swirls in the piston chamber. Without this low pressure area less air would be pulled into the piston chamber. A benifit from having back pressure is air entering the piston chamber from the intake side hits turbulant exhaust air. This further mixes the air and fuel which results in a cleaner burn. The better the burn the more power the engine will produce.

power = air and fuel

the more air and fuel in the piston chamber the better. You need to run a header unless your running forced air, and that another completely different way to make power.