By that, do you mean scavenging? Or is it something that the 900RR OEM headers cannot do but aftermarket ones do due to a different design?
900RR headers are designed for a radically different (carb) inlet through to exhaust port exit design, cam timing, and resultant torque curve, and positioning of peak torque and power on the rev band.
The idea is to maximize, and control, gas flow through the entire system over a range of revs, throttle openings and engine loads.
(Scavenging generally refers to simple exhaust gas distraction, but the required overall gas flow and control of it is way more complex than just scavenging in isolation. It is a factor, though. Scavenging is much more of an issue with two strokes.)
The header design is key and matching component to all the aforementioned.
Hence why they are such a mismatch to the particulars of the 919.
As evidenced by different mapping being needed to get the best out of a 900RR header on a 919.
For a stock 919 engine, the factory header will be very hard to beat.
An aftermarket single outlet header could be designed to give a bit more in peak power, but at the sacrifice of some torque and torque stability in the lower revs.