Interesting question. No, I have not done any measurements, but wanted to comment on something else.
The reason for the second question is that everyone who flies a single either knows the effect is real and material or has had some very exciting stall experiences !
I don't agree that "the" (P-) effect is real. I do agree that you need a bootful of right rudder on takeoff (US Lycosaurus talking here) but there are at least four different effects to which this can be attributed:
- Slipstream effect
- Torque effect
- P-factor
- Gyroscopic issues
I have no idea what percentages to assign to each individual effect, and whether one of these effects might be a myth anyway.