From the looks of that pic, that poor old 919 sounds, and runs terribly. If it was me, I'd put a 919 system back on, because the stock system was designed to work in conjunction with the engine, and state of tune it's in. Then if you want, swap out the stock mufflers for something like Delkevic.
The newest 919s are 12 years old. These bikes are getting old. I don't know who makes what, any more. I'd look at the Delkevic website. Their quality is unknown, but at least they still make stuff for the 919. If you still want to do an RR system, they probably have a mid, and muffler that would work.