I had not thought of the interventions you describe, so will have to look at them.
I could have just used the elbow to redirect the noise and not added the expansion chamber, small as it is.
I figured no change in power, but will see. Am guessing the large stock muffler does what it is supposed to do, regardless of the change, as the new pipe has greater diameter, but I am no expert.
I do know that if you install one of the very spendy power pipes, they apparently require a removal of the stock muffler and bolt right to the manifold, so I figured no HP change as the stock muffler remains. Might be better?
Copper will likely just dull to bluish metal and finally go green, but not sure.