Now the 5D2 will spank the M8 for resolution, dynamic range, color accuracy, speed, low noise at high iso, autofocus, and flash capability.
I have not tried a 5dII as they are not out in UK but I do not doubt the sensor will be pretty good. Im not sure the difference will be as great as you think though up to A3. I have tested both a Nikon D7oo and also the Sony alpha 900. The D700 does not match the overall image detail with the nikon glass I tried. The Sony with Zeiss 24-70 was absolutely amazing once you up the image size, but colour rendition not as nice as the M8 and quite honestly at A3 size not noticeably more image detail.
I do conceed the flash / auto focus / overall flexibility would score a lot of brownie points.
I still wonder if we shall see a digital R series.
Richard