[noggin]

Thanks for the information........ I tried this again at my end and Nvidia Shield simply wants to drive the Samsung JS8500 and 4k@60 all the time. Cannot even get the display refresh rate to change!
Must be something specific with the HDMI handshake between SATV<->JS8500 since I seem to be the only one experiencing this.

It is my understanding that Kodi cannot switch resolutions, say from 4K 60Hz to 1080p 24Hz and it switches refresh rates only for 1080p resolution.

That is true for mainline releases. However fritsch has added some code that will intelligently switch between 1080p50 and 1080p60 for SD/HD content that needs the higher frame rates, and 2160p24p or 2160p25 for stuff that doesn't, to allow the optimal modes for content to be used when you have a non-HDMI 2.0 4K output that doesn't allow for 2160p50 or 2160/p60 output. (This is not actually an issue for the nVidia I believe - but is for lots of Intel Haswell boxes)