Skip this thread if you don't have patience with graphs and numbers, but there is information in Sean Reid's white wall pictures that gives a pretty good picture of what the M8 firmware does to correct color vignetting, and what the sources of this are for the 28mm lenses that he discusses in his current draft article at reidreviews.com.
Here are two plots, for the Elmarit 28mm and for the Summicron 28mm, of the relative strength of the red compared to the average of the green and blue values along a line starting at the middle of a uniform white rectangle and proceeding to the upper right corner. In each case the lower lines are without any firmware correction, and the upper pair of lines are with lens detection enabled (these are Leica-coded lenses).
Notice the results for f/2.8 and for f/8 are pretty much the same. The Elmarit is a more symmetric design than the Summicron, and sits closer to the CCD surface. So it shows stronger vignetting. The firmware increases the strength of the reds near the edge of the frame to the same value (depressed by about 8%) by making a stronger correction for the Elmarit than for the Summicron. This firmware was developed before Leica recommended that IR filters be used, so it is the correction for the extra blue-green filtering that occurs at the edges due to the existing cover glass on the CCD. The white walls were shot with an IR cut filter added to the front of the lens. In principle, the red vignetting expected from a filter in front depends only on the angle of view (of the focal length) of the lens, and should be the same for the symmetric Elmarit and the telecentric Summicron. These graphs are consistent with that expectation.
After looking at the roughly equal size of the color vignetting corrections from the filter and from the cover glass, I would want to have the assistance of the firmware both with and without an IR filter. I think there will need to be a menu item toggling the correction for an IR cut filter off and on. Just turning lens detection on and off is too crude to give the best color fidelity.
scott
PS -- a few details. The measurements were made by taking screen captures of the four unlabelled white wall shots with and without lens detection in the current version of the article. I then folded my laptop screen flat, facing up and placed a marked piece of paper across each image diagonal to determine the locations to measure. (The last points are unreliable, because I couldn't measure a constant distance from the corner.)
I also have measurements of overall vignetting for these two lenses, which do depend on aperture as well as the details of the lens design. I'll discuss them after comparing with the uncoded lenses. There is no accurate theory for this, since it is measured after including the helping effect of the offset microlenses.