Not an expert..... but it's my understanding that any mods (within reason) downstream of the mouth/intake (i.e. exhaust) are handled by the stock computer. It can make allowances for "some" variance in fuel mixtures.
Simple exhaust changes
typically don't require a change in air/fuel mixture changes without other modifications.
Simply (and crudely) put:
Having a larger diameter pisser (urethra) doesn't allow you to drink beer any faster (or more of it), it still has to go through the whole digestive system. It only sounds different when it hits the porcelain of the toilet.
Flashes and aftermarket tuners will enrich the mixture "more" than the stock computer to allow for cooler exhaust temperatures and allow for more modifications that will exceed the stock computer's ability to compensate..... i.e. cams, intakes, head/valve work, larger pistons, stroke, injectors, etc... major stuff (the "system" mentioned above).
I don't know about the performance or sound.... other than the fact that my buddy has a '13 CVO. Sounded nice to me, but he eventually changed it and got a tuner..