We don't have one set up...yet, our Storm Lake extended threaded barrels just came in (we don't have them on the web site yet). Done correctly an extension will work but that money could be put toward a threaded barrel and you could put it back stock whenever you wanted to.
Sports South, a distributor in Shreveport, Louisiana, is selling the M&P 9 Special Ops, which has a factory threaded barrel and three high cap mags standard.
It is silk screened Spec Ops, which is stupid, no external safety, no kiddie key lock safety, no magazine disconnect, if your round is chamebered and you pull trigger if goes pffft.