I've had one for several years. I get very little POI shift on my 11.5" SBR, my 16" 6920, or my 18" Sabre Defence SPR. I get silly, unexplainable, ridiculous, won't believe it til you see it shift on my 24" Mossberg MVP - over a foot left and over a foot high at 25 yards.
I've never had any troubles with Barsto threaded barrels on various hosts. I've had them make me ones for Sig 226's and 220's as well as one for a Glock 34 and 21. I don't know if they make them for the M&P, but it's worth looking into.