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?