Since the late '70's and earlier '80s car makers started using tuned exhaust, and cold air intakes(CAI)
Tuned exhaust means at, and near, a specific RPM the exhaust flow's velocity creates a negative pressure at the exhaust ports in the head.
This negative pressure means the crank loses less energy pushing out the exhaust, it is pulled out, so more energy is available for the vehicle.
The negative pressure is created by using the correct diameter and length of pipes in exhaust manifold(header) to create the velocity and then dumping that velocity into a larger pipe(collector), this sudden drop in pressure, from smaller pipe to larger, creates the lower pressure at the other exhaust ports on that bank.
The lowest pressure can be "tuned" in the RPM range, most stock manifolds are tuned for mid-range having lowest pressure so the most engine power.
3rd party headers are usually tuned for lower end power, but "racing headers" are available which create the lowest pressure at higher RPMs.
Assuming you have a tuned exhaust then changing exhaust won't give more power, you can move where the power(lowest exhaust pressure) occurs in the RPM range, but power is the same.
This is also where the myth of "back pressure" came from, i.e. "some engines need back pressure"
How this got started is that people would remove the stock "tuned" exhaust, and put on "free flowing" headers, large diameter pipes.
Then after a test drive...."hey, WTF, I lost power, this engine must need back pressure!"
Result was correct but conclusion was wrong, the larger pipes didn't allow a high velocity in header pipes so they lost the low pressure and the power it added.
No 4-stroke engine runs better with back pressure, quite the opposite, 2-stoke engines do need back pressure
As long as you don't use a smaller pipe size for Cat or muffler it won't effect power, and eliminating "working" Cat or muffler won't effect power one way or the other.
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