My opinion on polity, to be clear, is that we see hints of several different forms of church polity in the New Testament. But we see no definitive polity which can translate into the exact polity used by any modern denomination. Also, IMO, I see very few of those pointing to the idea of local autonomous congregations. I'm sure I'm in the minority here on that view.
But that's how I see it. I think Baptist polity in the US is more of an American invention (and SBC polity a southern American invention) than it is any particular polity out of the New Testament. It fits the American idea of independence and self-autonomy as well as enlightenment values. It doesn't fit Paul telling churches what to do, the Jerusalem Council deciding things for the churches, etc.
And while Sandy may argue that SBC polity is New Testament polity it is going to be a huge stretch to identify Baptist Conventionism (word I just coined I think) and rules on denominationally owned school with any polity in the New Testament.