Sorry, never saw this thread back when it was started. I did a lot of B&W street work with the GRDIII. I played around with the high contrast B&W JPEG mode, but ultimately found it too restrictive. So about 99% of my street work with that camera was shot raw and processed with Silver Efex Pro. That sensor was pretty limited and raw was the way I found to get the most from it.
Probably little help and too late besides, but there you go...
-Ray