Daniel,
Congrats on the silver Lux. Looks to be a beauty.
Your question about the 21/2.8 somehow being less suitable IF (and that is a huge question right now) Leica does go to a FF for the M8(9), is a bit curious. Essentially, IF there is a FF digital M, the 21/2.8 will behave similarly to how the CV 15/4.5 does now on the present M8, as far as angle of view goes. With respect to composing things, is that you will probably have to use an external viewfinder, just as is needed with the CV 15 if you are really working hard to get composition, versus "guesstimating" and checking things on the LCD.
If the issue of suitability is related to edge details, and whether a FF sensor will be able to handle the wider lenses.....I do not think anybody has definitive information there, as there is no full-sized sensor in the M8 yet. Best comparison to date may be looking at the performance of these lenses on a film M. if the edges and corners are very sharp, that may translate quite well to a FF sensor if there are also good micro-lens alignments as are done on the present M8 sensor. Bottom line, nobody really knows yet, and those that might are not talking about it.
As an aside, I shoot a Zeiss 25/2.8 on my M8 and have no issues/problems at all. I did swap the lens mount and code it as a Leica 24/2.8, and I use a filter on it, but the results are excellent, in my opinion, and the camera firmware corrects what little vignetting and cyan corners there were. I do not have any point of comparison with the Leica 21/2.8, but others here may be able to offer their thoughts.
LJ