Pressure drop is to flow as voltage drop is to current
With all else equal, removing the resonator or muffler will reduce the restrictions in the exhaust pipe. This means that the ability of the exhaust headers to flow gases will increase, not decrease.
However, this does not necessarily mean that the engine will produce more power, since that is the result of several subsystems working together. So if the exhaust is bottlenecking performance, reducing exhaust restriction will improve performance until the point that something else becomes the weakest link. If something else (like air intake) is already the weakest link, a better-flowing exhaust system will have little impact on power.
Vehicles that employ wideband lambda sensors are sensitive to backpressure; they can tolerate deviations up to a point, but beyond that the engine may run too rich or too lean.