I have never been able to notice a one-frame sync difference (42 ms off). Probably because there are already ADR and SFX sync problems in the movies themselves, and a one-frame difference is less than that. A two-frame sync difference (83 ms off), however, is right on the edge of perception for me. It requires just the right sort of thing to be happening, but then I notice it. For other things, I don’t. Three frames is right out.
I wonder if the imperfections that come along with ADR are sometimes enhanced past the threshold where they are noticeable, with small frame shifts. Of course, at other times it might improve the effect. 😛