Exhaust systems changes are pretty well documented in what they do and don't do.
30-40 years ago auto makers just slapped on an exhaust manifold with minimal design effort.
With the advent of emissions and higher fuel prices all exhaust systems are now tuned.
But generally for mid-ranger power.
What a tuned exhaust system does:
It uses the exhausts Velocity to lower the air pressure at the exhaust ports in the head.
When you put a fast moving gas in a small diameter pipe and then dump it into a larger diameter pipe you get a pressure drop, this is called scavenging the exhaust flow.
Now the tricky part of the design is to get the lowest pressure at a specific RPM.
Auto manufacturers aim for lowest pressure in the mid-range
Header makers will usually have 2 models, one that moves the low pressure down in the RPM range, and one that moves it up in the RPM range(race cars want high RPM power band)
These do not increase power, they just move the lowest pressure around, so you feel the power earlier or later in the RPM range.
The myth of "back pressure"
This comes from people designing their own exhaust systems.
No engine likes or needs back pressure(not counting two-stroke engines, they do need it, lol)
People would slap on 3" pipes at the heads for "a free flowing system", and lose power, WTF!!!
"Engine must need back pressure", wrong conclusion.
The larger pipes at the heads reduced the Velocity in the system so they lost any possible low pressure from the flow, so they lost the power they had from the original smaller pipe design.