So far...... I've owned 16 bikes....and have had pipes on just about every bike. I've owned Akrapovic's, Arata's Titaniums, Remus, etc. etc.
I went through all the options with the 919....couldn't find a nice set of Sato's --- said screw-it.... picked up the single sided Two Brothers from Star Cycle. I'm EXTREMELY happy with the pipe. Great sound.....super light....and inexpensive. The kit even came with a Y pipe eliminator and a rear Galfer steel braided line.
To even better the deal.....the Power Commander website has a PCIII-USB map that is pretty damn close. Shortly after installing the pipe I did a dyno run with the sniffer..... not perfect, but not far enough off to warrent any extra tuning.
BTW --- the biggest power increase for the 919..... is opening up the exhaust, and getting a PCIII-USB w/ a good tune.
And no..... I was worried that the pipe was going to be loud like a D&D..... nope, sounds great and doesn't turn any heads.
They still sell for $356 at StarCycle.