Sigma have just released a firmware upgrade for the Quattro series and a new version of Sigma Photo Pro, version 6.2.1.
Yesterday I upgraded both software and firmware. There does seem to be some real improvement in shadow detail, but so far as I can see, there is no improvement in the clipped highlight department.
This image illustrates the problem well. It is a medium contrast image of a slightly messy kitchen work surface (mine - sorry!). At first sight, it is richly detailed and with excellent colour. But look again at the lemons and the tulip tips. The yellow of the lemon skin is completely bleached with jagged edges, as are the tulip tips, in a very unnatural looking way
If you then look at the following enlarged crop, one sees that there is some kind of etched outline detail in the bleached lemon skins. I have never seen this with any other camera and can only surmise it has something to do with the redesigned sensor layout.
One sees this exact same problem with blown out skies in landscapes, but with the added issue of false colour infill, for example, between tree branches.
You don't see this problem with the Merrill series.
Perhaps someone would be good enough to get hold of Kazuto Yamaki, President of Sigma, and ask him to focus his engineers' efforts on this issue before he wastes time and money developing new cameras based on the re-designed sensor.