Quick update.
I ordered and fitted an Apex SDI barrel into my M&P9 4.25" Performance Center. Here are the results. I shot from 20 yards (max distance at my range). I didn't have sandbags so I braced my wrists on my soft range bag which was not a proper rest but was as close to a reasonable approximation as I could put to use. Same ammo, same day, same gun, same distance -- different barrel. I shot a few different types of ammo during this session and the results were similar across the board.
Factory OEM barrel:
Glorious ~10" spread!
Best OEM group of the day
Apex Tactical Barrel:
I don't work for Apex. I don't have friends that work at Apex. I didn't receive this barrel for free. Nothing shady is going on here. YMMV.
So all those S&W apologists who immediately jump to blaming the shooter despite whatever evidence is presented to the contrary can put that in their pipes and smoke it. Practice is no doubt going to make someone a better shooter. However, I can practice flapping my arms all day and all night; but no amount of practice will make me ever take flight. Just how widespread the problem is would be anyone's guess, but based on the years of complaints I doubt I'm the only one that has a genuine issue.