Here are the the two flat field images, shot at f/11 with a diffuser against a bright sky, the M9 first and then the M240.
|M9 flat field image|
|M240 flat field image|
The average amount of light loss on all four corners with the M9 is about 1.54 stops. The average amount of light loss for the M240 is about 1.44 stops. Thats just sampling the most extreme pixels and shows a slight improvement.
If we take the center line and plot the light loss we get:
Again, the difference isn't huge but its pretty clear that the M240 is better not only at the very edges but starts to have its slight advantage at about quarter frame from the center. If you do flat field correction in either Lightroom or Capture One the improved noise floor on the M240 should give it a further advantage, in all making it a better tool for wide angle rangefinder photography.