The pressure differential when the exhaust valve opens doesn't depend on a few lbs. per square inch of mercury from "backpressure", since the initial exhaust pressure is perhaps 50 psi.
There is a long-standing tendency to conflate all the effects of an exhaust system. There are at least 3:
1. Systemic pressure (measured in the pipe before the muffler). This increases pumping loss, but not necessarily the conditions at any individual exhaust valve.
2. Local port pressure (directly after the valve seat). This is where backpressure is always detrimental, since it frustrates overlap flow and chamber purging. It's frequently the result of 3. (below), rather than 1. (above).
3. Multiple wave reflections from all valves opening, and changes in cross-sectional area including the muffler and tailpipe.
Factory exhaust installs are carefully designed and arranged so that any accidental positive wave arriving at an exhaust port during overlap is damped as much as possible. Something as small as juggling the pipe lengths before and after a muffler, or changes in muffler volume will make this better or worse - it's not backpressure.
