I have read this Thread with interest as my wife recently purchased a S&W M&P 9mm.
My background is in Engineering and QC/QA.
First, without the use of a machine rest such as a Ransom, all accuracy testing is really inconclusive and subject to the ability of the shooter and perceptions of the shooter.
Second, all the speculation of the cause of any inaccuracy is meaningless BS without a knowledge of how S&W designs, manufactures and assures thier products, IF thier is a problem to begin with.
I have seen this sort of problem identification and speculation get out of control before on the Internet and result in an 'Urban Legend' that seamingly never dies but get repeated 'ad naseum'.
I have still not seen objective information presented that there is a problem and certainly not any information about causation.-Dick