Remember you will not get a big increase in power on the 919 by adding some cans.
I don't understand the issue people have with Two Bros - their modern cans are well made, good looking kits that satisfy the Hornet owners desire for lighter cans, more noise and a cooler look. They are well regarded in New Zealand.
Interestingly I found data a while back on the net that the 2Bro's single pipe option gave the highest power/torque increases on the 919 - probably logical really as it removes the skinny little Y pipes that the twin system uses.
Compared with something like a J&D which looks like it was made from a old vacuum cleaner they are impressive.
The much revered Sato mufflers have issues with the cores rotting out - unacceptable for a very expensive set of cans. Plus from whats posted here it is debatable if they are doing much 'muffling' as they tend to be described as very loud! IMO highly overpriced.
And bang for buck there is Yoshimura - a pair of tri-oval cans for under US$450 amazes me - high quality slip-ons from one of the top names in the industry. These exact same pipes will cost me NZ$1600 if I buy them from a dealer here!
At the end of the day they are really much the same on a Hornet 919 - minimal power increase, more noise, less weight. Have a look at them all - all run 50 mm (or close) perforated inner cores in almost identical lengths with packing to quiet them down. 50 mm mid pipes to attach to the stock Y pipe (same as you stock mid pipes). The rest is cosmetic.