Hello Quentin,
Good to see you on this thread again. There is no doubt, as demonstrated in this thread, about the superb image making of DP2M. This thread is literally crammed full of high quality photographs that put many high end cameras to shame.
You've mentioned the upcoming sd Quattro HD and your hopes for it. I'm afraid I have my doubts as my experience with the dp0Q shows it to be a Jekyll and Hyde camera. Occasionallly, it'll deliver excellent results, possibly better than the Merrill but more often than not, the images are disappointing. The sd Quattro appears to be the same. Here is a link to a Japanese photographer using the sd Quattro that I hope qualifies this statement.
CASE OF SIGMA | sd Quattro ?????????? Vol.02 | -photo.yodobashi.com-
It looks like the sd Quattro will only work well, like the dp0, within a very constricted set of parameters. Perhaps the HD will be different. Who knows?