unless there is a cross-over pipe between the outlet of the exhaust manifolds, there is nothing that will stop the output volume of air if the cylinders are firing, only the pressure at certain places will change. The internal pressures of the cylinders after combustion is so much higher than exhaust back pressure it's not very consequential.

Check out this web page. It gives some good advice about backpressure and how to measure using the O2 sensor port.

If the engine 'runs smoothly' no 'ch chugga cchu ugga' just low performance than I would guess more likely to be backpressure problem, but if all 4 cylinders firing and no cross-over pre-cat than if not even volume out both pipes not even volume being created from burning, so previous line of reasoning prevails.