I believe there should be no loss of quality - just a file that will limit the size of the image.
Since I don't own a 5D2, I asked about this same question of Canon regarding my own camera, a 7D - the full size RAW files from a 7D are about 25MB...the mRAW files are listed as being on average about 10MB - I was told that the quality would be the same - no loss of detail or color, etc....but it would be more like as if I had shot with a camera like the 40D which has 10MP and creates full size RAW files of 10MB.
Hope this helps - and I hope it's accurate since a lot of what I do would be better for me if I had 10MB RAW files - easier and faster to post process and for stuff like youth sports, where there are very rarely calls for anything larger than 8x10", I'd at the very least be able to use less storage when shooting (by more than half).
Peace,
D.