I got my pencil out to see how big the p-effect should be. I made the following assumptions
300 hp engine
84 inch prop
53 inch effective diameter (the distance we between the point we assume is the centre of effort on each blade)
80MPH forward velocity,
10 degree angle of attack of the aircraft (say 13 degrees for the wing)
400 MPH blade velocity at the centre of effort
20 degree pitch (resulting in a blade angle of attack of 9 - about 2.5 times that in cruise)
This seems to result in a 650 N-M torque.
Something else I noticed was that at cruise (say double the speed), if you temporarily generate the same 10 degree angle of attack (which will make you go up at an astounding rate), the torque will only be 50% because the thrust = power (constant) * velocity (therefore the thrust of the engine will be halved at double the speed - assuming constant power) and the rudder effectiveness will be increased by a factor of 4 (lift being proportional to V^2), so the rudder deflection required to counter act a 10 degree angle of attack at cruise should only be 1/8 that at slow flight (just considering p factor here).
I am further dubious of the slipstream effect as wings (which are all prop blades are) don't normally impart a large forward velocity to the air, which in a prop would be necessary for the rotational effect argued in the slipstream case.
They do cause a significant tip vortex which would seem to be shed symmetrically around the aircraft and I think (if I have the mental picture correct) these would marginally slow the airflow over the left side of the vertical stabilizer and increase it over the right).
As a final note - the engine torque will be 'by definition' equal if the high speed cruise is done at the same power/rpm setting as climb out. 'High Power' was specified in my example to eliminate the differential lift (and hence drag) that is without doubt required to compensate for different engine torque in flight.
As Silvaire1 indicated, gyroscopic effect normally refers to the yaw generated by pitching a rotating mass about a perpendicular axis.