I would suggest that the 'P' effect is very real - consider the issue of critical engine on twins.
For a discussion on this (including other factors in addition to P effect) see:
Critical_Engine_1.pdf
Just my ha'penny worth (which is probably an accurate judgement on it's worth).
OC619
P.S. Heard during a briefing to someone converting to a Tiger Moth (anti-clockwise prop)
Q: Which way does a Tiger Moth swing on take off?
A: It doesn't if you're doing it correctly