This shows the actual L*, a*, and b* values when printing a b&W image using ABS and standard RGB color managed from a large 5.5k patch set with extra neutrals. The principal differences are that the transitions are much smoother between adjacent neutrals with the ABW and vary a bit more printing using a color profile.
The charts show rendering from L* 4 to 95. The blue lines are ABW mode.
The first chart is the L* out v in. This was done for ABW using a custom LUT. The RGB managed (orange) lines are from using a standard ICC profile. They are very close so the second chart shows the difference between requested L* and printer L*.
The third chart shows a*, and the fourth chart b*.
Also included is a photograph, processed scene referred, of printed images. The first image was printed with ABW, the second with standard color management. The third and fourth are the same but with saturation greatly boosted to bring out the color shifts. The printed gradient image is from using Photoshop's gradient tool and is not linear but the approximate L* on the image (Photoshop Info Tab) is reasonably accurate.
The most striking difference is the relative smoothness with which the a* and b* change across the tone curve for ABW. The 9800 raw device RGB color response is rather lumpy and ICC profiles, with only 36 LUT spacings, can only compensate for the average when they rapidly change.