Looking at this myzmanim.com chart for Brooklyn, NY on 01/21/2013 it gives me the following Zmanim for Mincha Gedola.
30 fixed minutes after midday 12:37:27
Gra & Baal Tanya 12:31:53
Magen Avraham Using "Fixed 72 minutes..." 12:37:53
The Zeman for Chatzos in New York according to Rabbi Moshe Feinstein in NY is 11:56, so that would make Mincha Gedola 12:26.
Now I have observed 2 Shuls recently that started praying as early as 12:20. If I remember correctly even Ashrei is not supposed to be prior to the Zeman of Mincha Gedola. Are there any other authorities that have an earlier Zeman for Mincha Gedola?