I would rather be slightly above plane than anywhere below....when I get below plane I can lose PP3 and hit the dreaded hosel rocket.
A lot of great ball strikers are on the elbow plane and most still are double shifters....some shift even more.
A lot of tour pros start out on the hands only, shift through the elbow plane and land on the turned shoulder (some, like Mayfair, Hoch and Couples, get up to the Squared Shoulder) and then come back down and hit the ball on the elbow plane
I have asked about this before ..what did homer k consider to be correct elbow plane Perpendicular forarm to spine angle at impact would seem to be true elbow plane rather than than a larger degree angle