Clearly, the one shown there with Qxh5+ Rxh5# can be easily turned into a s#2, by e.g. B-e2-f3, Qe2 from White and c6, Q-a5xd2-d1 from Black. 9.0 moves if I added correctly. Someone care to beat this, or even construct a helpgame-s#3? (This might be far trickier than a #n counterpart!)
Addendum: This example doesn't work as the Black queen vacates d8 for the Black king.