I don't think that Smith + BM consistently has 5 Provinces + 1 Duchy on turn 15.
I played it a little, and I think that the consensus that is developing is that ehunt's 5 Provinces by turn 13 was anomalous. I didn't get that good, but I did have 5 Provinces by turn 15. I'd be willing to believe that there's a skill-component to this as well -- I don't think that entirely mechanical "put cards on the NV mat until you can guarantee yourself 4+ Provinces in one turn" is optimal. In my game, I put a couple of Coppers on the map early on. Well, Coppers aren't useful on your mega-turn, so I yanked them back off the mat in order to get an earlier Peddler than I otherwise would, which seems like it was useful to me (sure, my deck was less thin, but I got my Peddler). What if you can do 3 Provinces in one turn, is that enough or should you wait? I'm not sure, but I suspect that play need not be entirely mechanical here in the way that, say, "Bishop + Chapel -> 5 card deck trashing a Province every turn" is mechanical, or "Workshop/Gardens" is mechanical.