The F800gs Ecu will accommodate most any muffler. The oxygen sensor will indicate when the fuel ratio is stoich (14.7 A/F) and the Ecu will supply a map to maintain that in closed loop. Open loop will be richer.
If you live in the US (fill in your profile) an after market part will not void your warranty unless it is the direct cause of whatever problem you have.
Aftermarket mufflers should not cause any problems for the bike. There is a rub, because the dealer initiates the warranty process. I would suspect a lot of engine problems might fall into that gray area if he so chose. How good is your dealer?
BMW has officially approved an Akrapovic aftermarket muffler. You can tell because it cost $981
. BMW probably gets a piece of the action for recommending it. It is not that special, but in some European countries they cannot add aftermarket parts willy-nilly. They can only add parts approved by the manufacture.
I have a carbon Leo Vince and like it.