Will it has LLDV as good as the UGOOS?
On LLDV: Hades' build has it (setting off by default, and it applies only when the display offers DV solely as Low-Latency). I tested RC4 on an RX 7600 against a Homatics R 4K Plus, through a VRROOM with an LLDV EDID, and both looked natural. The tone mapping is Hades' own rather than Dolby's, so it isn't identical: moderate colours matched a reference composition, saturated ones came out brighter. CroqueMr's build had no LLDV when I last tested it (RC2).
I haven't tried an H 255 (780M) or Plex, so I can't say anything about those or about bitrates.
I was wondering whether extending the kernel patch to support DP-to-HDMI connections might be technically feasible.
Thanks for the offer, but I won't be writing this one. From reading the driver, only a passive DP++ dongle could work with a small generic change, because the GPU then sends a real HDMI signal. Active converters are blocked by the driver itself (only frame-alternate 3D is allowed, "bad infoframe translation"), and the 3D infoframe isn't sent on DP at all. A 4K DP-to-HDMI cable is often active, and both adapters I own are active, so I can't test the passive case. If CroqueMr wants to try it, the place to start is the HDMI-only stereo_allowed check.