You best performance gains would be a full system, air filter, and FI remap (Power Commander if on is avail for your bike). Dyno the bike before doing the mods, and tune it on the dyno after the mods. Going to cost a little money, but that's the best way to ensure gains from the money you spend.
If you just do a slipon your FI will probably compensate for the small change, but performance gains aren't garaunteed.
Personally for you bike, a full system will remove that undertail look of you 919 (side mount pipe) and it will probably be pretty loud. If I had your bike, I'd go with a nice set of mellow undertail slipons.