Fuji X-20 compared to Nikon D50

Depends on which FF sensor you're talking about. X-Trans to a Canon 5D FF sensor, yes. It is not even close to a FF sensor in Nikon D3s. On D3s, ISO 1600 is like X-Trans ISO 400. My D3s puts my X-Pro1 to shame even at ISO 400.

The real advancement here is actually the core Sony sensor, using a 22um process and with A/D converters embedded on the sensor itself. What Fuji provides is merely a different CFA. With the extra 2 green pixels in a 6x6 grid, san-AA filter, the X-Trans gets you a slightly better luminance signal (hence slightly lower noise than a Sony Bayer sensor), but in return, the red and blue colors are more muted with higher ISO on the X-Trans. There is no free lunch.

Now, it depends on who makes the X20 sensor. I don't think it is by Sony, so it is likely to be an older generation design with the X-Trans CFA. This is why we do not see a significant high ISO improvement on the X20 sample photos so far.

In fact, X20 will need to go up to ISO 800 to achieve DR400%. A sensor's native DR is reduced with higher ISO. Where as X10 can do it with just ISO 100 in M size capture.