So I was tempted to pull the trigger on the 55KS8000 and sell my 55X8500B, but I'm glad I didn't. I went into Harvey's today and checked out the 4K HDR demo on the KS8000. It does look very bright and crisp with great quality and I really like the design, however I can't justify spending more to upgrade to an HDR TV. I would only upgrade the TV to a 65" if I was going to, but I feel HDR just isn't worth it and can't tell a lot of difference. Some of the colours were oversaturated and looked artificial. I went back home and turned on the X8500 and I'm still amazed in how it looks, just turned the colour and backlight up a notch and it looks pretty damn close.
imo if you are upgrading from a non-full HD/full HD TV, then going 4K HDR is definitely worth it, but upgrading from a 4K TV to a 4K HDR isn't, unless you upgrade in size by 10 inches or more.