Road legal aftermarket cans are very similar in performance and sound to the stock units. They are mostly for appearance and do not require any changes to the EFI, emissions, etc.
Race or non-road legal cans are lounder than stock, but offfer very little performance gains. Again, no other changes are needed. Just bolt them on and ride.
Full systems (from the headers to the mufflers) do require changes, both single canister (4-2-1) and dual canister (4-2) systems. A Power Comander will be needed to remap the EFI. The PAIR valve emissions hardware must be removed and the holes blocked off. Failure to remove the PAIR will result in backfires, shooting flames, and possible engine damage. Full systems are generally much louder than stock and are almost certainly illegal in Europe.