I've done some comparisons of the DNG file with lens corrections on/off. With the M9 and M Monochrom, these are mostly corrections for light loss at the corners. I do not use lenses that would cause a color shift in the corners, the "Italian flag effect". There is no correction for lens distortion. The processing applied is minimal. On the M8: I have it turned off and have not used it since buying in 2010. I've used the Jupiter-12 and Nikkor 2.8cm F3.5, these being the most "strenuous"- have not seen an issue.
On the M Monochrom- my "trick" is to apply the Gamma curve and output a 16-bit (as opposed to 14-bit) pixel. The DNG header is updated, and the file can be opened in Lightroom.
The "test"- I like the Gamma DNG files better than the straight raw ones, find I do not need to apply much post-processing to individual frames. I also restore underperforming lines due to bad pixels, rather than interpolating over. Just annoyed me to see a bad line on the M Monochrom and know the signal was still there.
The M8 DNG processor- I have a custom one for using the camera with an orange for color IR. With the Orange filter the "Blue" channel is Infrared. There is enough of a signal to equalize it with green and red, gives an "IR Ektachrome" look. I used Fortran code written in the 1980s for the base, originally written for custom sensors. I stopped working on those in 1993, switched over to optical networking.