After I used the then newly added M8.2 a few days after using the R-D1 for a while (loved it and strangely still keep it), I have written a quick brain storming list on my immediate thoughts on the M8.2, which is still partly valid as of Today.
You can find the list here:
http://www.teknopunk.com/articles/about_the_Leica_M8.2.html
I struggled a few weeks (!), to get happy with a new workflow and processing with the Leica files and almost gave up on this during the process, so different are the files (especially for exposing in low light and potential push processing).
Seeing it from Today's perspective, I say, once, you learned the M8, you will leave the R-D1 alone at some point.
The M8 has better sensor output - sharper, more detail (not just a higher resolution).
The M8 feels entirely different in handling at first going in the way, when being used to the R-D1 (no shutter cocking lever, no direct controls for ISO, different AE Lock, no shutter speeds in viewfinder in manual exposure, etc…)
The M8.2 is faster than the R-D1 (if you must dive into the menus, the M8 feels responsive, while the R-D1 can be sluggish).
Battery life is a lot better with the M8 (I usually shoot 300 - 400 shots with the M8, which is about 100 - 150 more than the R-D1).
The files are actually smaller, which is very nice (a bit over 10MB each raw file).
The viewfinder and especially the rangefinder is a lot better, although unfortunately no 1:1 finder (much better contrast, nice, that the focus patch stays centered, not to speak about the most important fact: better precision, allowing long and very fast lenses, to be spot on reliable, once set properly).
The M8 is actually less bulky (lower top) and a lot cleaner in design (less hanging, while stuffing or pulling into/ from bags/ pockets).
The lens mount is more precise, heavily damped lenses like Noctilux, 90 Cron, Nikkor 105/2.5 LTM fit without play and do not wobble around in the lens mount, when adjusting focus precisely.
If you ask me, the M8 is an adequately priced upgrade on a R-D1, judging from Today's prices.
The R-D1 is still a very nice camera, especially with it's still unmatched out of the box black and white high ISO look.
The M8.2 files need more and different post processing, which is no issue, once, you have your workflow and software down to scratch and fly through the files.
The M8.2 enabled one thing, I could not dream of with the R-D1 - shooting motor sports with a 135mm lens, which I didn't do with the R-D1 (RF precision + lower res for potential crops, as reach is not sufficient).
I would say, go for a M8, make sure, it is top (usual issues, than could be there and can be EXPENSIVE) and you might find yourself using the R-D1 less and less.
You could also just sell the house and go for a nice second hand ~4000 EUR ish one year old M9 with rest warranty from private, making sure with Leica Customer Service, that you can overtake the warranty and have any potential flaw fixed.
I can tell you, that the M9 is worth the money over the M8 - I just added a M9 and the files above ISO640 are crazy, compared to the M8.
The R-D1 has finally found it's master, when it comes to high ISO files.
Btw - the M8 does not need any lens coded. Extreme wide angle lens (with strange angling rear elements do vignette less wild than with the R-D1).
Any potential color issues can be fixed in post to a satisfying extend.
To my surprise, I found, that particular lenses, that performed poorly on the R-D1 were pure bliss on the M8 (less aberrations, less fall off, sharp from edge to edge, namely the Leica 35 Cron ASPH).
