MinGW still only has good old stable 3.4.5. There's not much reason for them to update it.
When I looked at the 3.4.5 compiled SHA disassembly, I couldn't see any room for improvement at all. I can't imagine how 8% more could be squeezed out of it. Is it possible Windows could have 8% more overhead? Not making system calls or anything, just plain busy computational code, could task switching and other housekeeping operations take away that much?