SPP with the sd Quattro seems to work somewhat differently for monochrome images. It still has the color mixer but it appears as if the other layers still may have some effect on the top layer. You can get pretty good results at higher ISO in monochrome though.
Boring test shot... lightroom XY comparison of SPP Blue only on the mixer vs. default at 1:2 in lightroom. ISO 3200 from sdQH.
If you want to be sure you are using top layer only use RawDigger and then export the blue layer as a TIFF file.
SPP also has an option to bin the top layer above ISO800 to reduce noise but then you have 1/4 the resolution.