Pavel: a big thank you for making the originals available. I've downloaded and printed them all (on A3) using exactly the same settings in Lightroom 2.0 and I'm astonished at how much more detail the GX200 captures (look at the sack the ficus pot is in and the woodgrain on the desk in lower left corner!). It's almost as though the GX100 photos are out of focus, but I don't see any plane in sharp focus so I don't think that's the case.
I notice the GX100 photos are also about 1/2 stop faster shutter speed and about 1/2 stop underexposed. Strange the difference in metering. I notice this 1/2 stop difference allows for more shadow detail in the GX200 (look at debris in the ficus pot) but with no extra blown highlights. The dynamic range seems pretty similar but maybe slight edge to GX200.
As for the ISO comparison, in prints of the same size I give the edge for ISO 64-200 to the GX200. ISO 400 shows the GX200 to definitely have more colour noise, but also more details. I have a slight preference for the GX200 at ISO 400. By ISO 800 the noise is fairly objectionable in both, but moreso in the GX200. By ISO 800 I'm preferring the GX100 images. ISO 1600 wouldn't be usable for colour work in either case, but the GX200 is significantly worse.
Out of curiosity I also printed B/W images of the ISO 800 files. Here I do prefer the pop that the extra resolution and detail in the GX200 images brings to the image. Colour noise isn't an issue now of course, but there is still a little more noise in the GX200 image. In addition, I prefer the way the grain looks on the GX100. That said, on the balance I slightly prefer the GX200 images in B/W at ISO 800.
This makes it tough to decide whether or not to upgrade from the GX100. I was hoping for more usable ISO 800 colour images for indoor photos of the kids without flash. Otherwise the GX200 is a pretty compelling package, I especially desire the RAW buffer. Hmm... How long until the GX300?