I am finding that SPP Monochrome can give some very good results at high ISO. I have gone as high as 1600 so far. I push the colour mixer all the way into the blue channel, I don't
add grain in SPP. In LR I adjust and reduce noise carefully, then add grain and adjust Clarity. It looks very "natural", very film like. It means the Sigmas can be used as B+W street cameras, something I need to investigate more when I have the time.