So I have been reading your contributions (thanks very much !), and researching different options. Here is a brief summary:
1) Sony Alpha 7R + 24-70 f/4 + 70-200 f/4: total weight: 465g + 426g + 840g = 1.7kg. Image quality: oh yeah ! This does fit into my grand scheme of having a very good landscape body for my Canon gear (I do wildlife photography too, and then I take the Canon bag, with the 24-70mm, 70-200 and 500mm f/4 and 1Ds III body - and no, that's not necessarily lightweight). But the Sony combo is a bit on the heavy side, especially compared to the featherweight option 4.
2) Canon 7D+18-200. This is because I already have a 7D, so adding a super-zoom would be easy. Weight: 820g+600g = 1.4kg. Image quality: so-so, at lead photozone isn't very positive about the zoom. But the sensor is probably pretty good. Improving the IQ would mean schlepping either the 24-70 and 70-200 f/2.8II (that I already have), and that's a no go.
I could also combine the Sony A7R with the Canon 70-200mm f/4 for example (which I don't have, but would allow to not invest too heavily into Sony, in the hopes that one day, Canon gets it's act together).
3) Olympus OM-D E-M5 + 14-150 mm: 425g+280g = 0.7kg. Image quality: good to very good. It seems the zoom is pretty decent. I could split the focal range into two lenses instead of the super zoom, and that would probably significantly increase IQ (at a moderate cost in weight). But it's more hassle and an increase in bulk.
4) Sony RX-10: 0.8kg. Doesn't look that attractive anymore considering the option 3 above. It's heavier, less zoom range, but I suspect the Sony lens is better that the Olympus 14-150mm. Any idea which would be sharper corner to corner ?
I had a chance to have a look at 3) and 4) in a real store yesterday. The build quality of 3) seemed a bit flimsy compared to 4) - but maybe that's what you get, when you want really lightweight. I didn't really like the powered zoom of the Sony. Perhaps Sony's viewfinder was a tad better than Olympus'. The Olympus was really lightweight, and the zoom range quite nice.
Choices, choices ! :-)