OK I see where you are depending on the mass of cut material to be carried around in the cavities between the rotating knives then being re cut as the centrifugal motion and slight vacuum created by the rotating knives continue to recut the shredded material until it becomes small enough to pass through the screen.
All the more reason why I would think increasing the degree of wrap of the pipe encompasses the spinning knives while making a screen which would share in the shearing of materials could improve on the output. Elongating the holes in the screen may or may not be well served.
I have a few hundred lbs of drink bottles that I need to granulate so maybe one day I will test my theory. Possibly by making a 3 rotor design using counter rotating carbide tipped saw blades for the initial shredding and the rotating knives passing by a fixed knife and screen for the granulation process