[noggin]

I can't say anything about HDR10, don't know . But the 4k 264 & 265 sources I've tried has worked flawlessly.

Ah - my understanding is that all 10-bit content is truncated to 8-bit. This will introduce banding on SDR and HDR 10-bit stuff - but this will be more noticeable in HDR content watched in HDR (you'd have to force your display into HDR10 mode I think as the C2 won't flag that over HDMI as it's an SDR-only box). I don't know what tone mapping the S905 builds to to convert H265 UHD HDR content to SDR. That would be my concern with UHD content on a C2 - unless your content is all SDR (UHD movies are likely to be HDR now, but other content may still be SDR)

I want progress, I want new hardware, but the last years s905 has dominated when coming to price/performance ratio.

I'm holding my thumbs for s912. But s912 should be the natural successor, right? If it will be supported in newer kodi releases? 

  
The S905X/D (and now S905X2?) are also successors - as they support HDR10 and a 10-bit decode and output path I believe (some rumblings that there was an 8-bit dither but @wesk05 appears to have demonstrated good 10-bit output) AND have proper Linux GPU driver support which will mean they continue to work in future Kodi releases after Leia (Kodi 18)

The issue with the S912 is that unless the Open Source GPU drivers arrive, the current Krypton/Leia builds that rely on hacked Android GPU drivers (LibHybris?) won't be a solution for Kodi 19 when Kodi moves to the new Linux graphics driver model.

The issue with x86 Intel GPUs at the moment is that under Linux, there is not a huge amount of HDR output (and 10-bit decode?) support - and you need a Core-series CPU (not Celeron / Pentium) for HDR output (Intel use HDR as a premium option). As a result HDR output is only available in Windows at the moment on Intel GPUs AIUI - and in Windows it's fixed at the OS level (I don't know if dynamic HDR metadata is supported)