Unless there are other running complaints there are only a limited number of causes for the loss of power at the speeds you are talking about. They are, in no particular order:
Weak ignition caused by both coils going bad, high tension wires / plug caps old and arcing, weak or intermittent signal from the ECU, or badly worn plugs. Any of these would cause other running problems.
Restricted exhaust. Unlikely with the system you are running.
Low fuel pressure / delivery during high demand running. This can be caused by a fuel pump on its last legs, a clogged pump inlet screen, a partially clogged fuel filter, or all of the injectors contaminated with old fuel varnish. Of all of these the most likely is the fuel screen or filter, both of which are in the tank.
Restricted air inlet. Either the filter (unlikely with the clean K&N), or the inlet flapper valve, there to decrease the inlet noise in order to pass noise level requirements. Of these I'd say the flapper valve is the most likely, and the easiest to address. Go here: https://www.wristtwisters.com/forums/...ion-12219.html
for a more or less comprehensive description of the "flapper mod". If it's stuck closed by a malfunctioning solenoid valve it would definitely restrict air flow enough to cause the symptom you describe.
Just for grins I took my 919 out for a ride and ran it past 100 starting at about 60 or so in 6th. It took just over 3 seconds to blow by 100, and was obviously anxious to keep right on accelerating. At that speed the motor is as smooth as a turbine. A while back I laid down on the tank and let 'er rip -- it showed signs of running out of breath at a calculated 134.6 MPH, but up to that point it was quite anxious to keep on accelerating. It's definitely no slouch.