x850 is trash atm for HDR, input lag is almost 100ms. X800D can stay consistantly low in 4K+HDR gaming, around 33ms, color is even better than the much higher end samsung ks8000. It not the great screen for HDR however due to the lack of local dimming and peak brightness is low. But it does have a 10bit panel that should take advantage of HDR. Avoid low end Samsung, they are nothing like their higher end, mostly trash too.
I have the xbr850c and I just got the ks8000 this week. I was expecting the ks8000 to blow the 850 out of the water, when it came to HDr. Using the Revenant on a Samsung UHD player. I would say the difference is minnimal. Yes the Samsung is brighter. But not to the point of making the picture better. The HDr highlights mainly fire in this movie pop on both tv sets.
Actually because in my current setup the Sony handles color, contrast and motion better then the Samsung, you actually see more detail on the 850. The Samsung over saturates color so much that on bright scenes it will look more vibrate, but the Sony will actually process more shades of the individual colors.
I have only had the Samsung for a couple of days and can probably adjust it to more of my liking via calibration.
I don't have the Xbox one s so I can't test games on either tv. But when it comes to HDR on the Revenant at least the difference between 282 nits verse a min of 542 on the Samsung does not make much difference.