Relative to EFI remapping, I posed this question to the service head of a major European KTM dealership when I ordered the Akrapovic silencer. I would suspect that this advice would apply to systems by FMF, Leo Vince, et.al.
His response was that remapping was not absolutely necessary, but (in the case of Akra) it would increase the HP and torque slightly. For me, as long as the bike delivers power equivalent to (or slightly better than) stock, I'm good with that. For my type of riding (gravel roads, primitive dirt roads, etc.), more horsepower is somewhat irrelevant -- just more spewed gravel...hehe. I've lost a few pounds and gotten rid of the "camp stove" heat in the process.
His instructions were to mount the new silencer, start the bike when cold and let it idle for 15 minutes with NO throttle input to let the EFI sort itself out.
Here's some close-ups of the Akra unit.