[noggin]

I'm using LibreElec which doesn't have full HDR support in Kodi Leia (or earlier). LibreElec uses tone mapping to display a 'correct looking' image but its not displayed in HDR.

 Do you know if there are any comparisons between 'full HDR' and 'tone mapping'?  How big of a difference is it? 

Tone mapping is the process of mapping HDR video to SDR (and also - in some cases - mapping Rec 2020 wide colour gamut to Rec 709 HD colour gamut)

It won't be HDR - so you lose the very reason for having HDR in the first place?

UHD HDR that has been tone mapped will be SDR - and in colour and dynamic range terms probably not look as good as an original HD SDR grade (Where the colour grading has been done with an SDR destination in mind). The only benefit over HD SDR will be improved resolution.