The effect that you guys are talking about with the warbirds (or any single engine aircraft) is from P-factor not slipstream effects on the rudder.

The p-factor is caused by the difference in angle of attack between the ascending and descending blades of a rotating propeller blade. Specifically, in a "nose-up" situation where the propeller disk is inclined to the flight path, the descending blade has a higher angle of attack relative to the ascending blade. The propeller blade with the higher angle of attack will provide more force.

P-Factor does not really come into play during 3-D flight since the forward motions are small and there is really not a difference in the angle-of-attack of the propeller during flight.

I could be wrong though, as I am sure someone will point out.