I have detected the "watercolor" effect from Fuji files, but it depends on how you process them and the size you view them at. I process with Lightroom and at 100%, you can definitely see it. I've played around with Photo Ninja and it doesn't happen with that. But at the sizes I tend to view and print at, I can't see it regardless. So, to me, it's a non-issue. And you'd certainly never see it at the sizes you posted at here. I'd still say the Df sensor is better than the Fuji, particularly at high ISO and particularly if you take a bit of Fuji ISO inflation into account. But, as you say, the Fuji definitely holds it's own - I decided to go with Fuji after shooting extensively with the Df. Not because I thought it was better, but because I found it close enough to as good. And comparing Fuji's excellent primes against the smaller, slower Nikon lenses I'd have been willing to use with the Df, the Fuji more or less matched it even in low light. Different story if I'd been willing to shoot with the big fast Nikon primes, but I wasn't.
I'd agree that Fuji has come a long way since the XE1, but NOT in terms of sensor performance. In terms of AF, MF aids, operating speed, features, EVF, yeah, much much better than the XE1. But the sensor is basically identical minus a few pixels that are used for PDAF in the newer bodies and weren't in the bodies before the XE2. And they have released a few more lenses since the XE1 and the new Fuji lenses are incredible - even better than the original three, which were already pretty great. But in terms of sensor performance, they're basically the same...
-Ray