I needed a new GPU for Divince Resolve as my existing GTX1660TI just did not have enough VRAM for 4K editing. I could not get a RTX3080 for love or money so after selling a kidney I got a RTX3090. Works well for Divince Resolve but.... how does it go with MC for video playback?
Short Answer: Overkill and Overpriced for a HTPC GPU
The good news, is you can crank up MadVR's settings to MAX for all normal UHD BD without issue including HRD Tone Mapping.... but,
Longer Answer: This mighty card is still humbled however when playing 50/60fps UHD HDR. There is only a couple of commercial releases in this format (Billy Lynn's Long Halftime Walk, and Gemini Man) but I've now a Sony FX6 shooting UHD HDR @ 50fps so playback of this format is of interest to me. I had no issue in playing this HFR HDR UHD material with Medium Chroma Scaling (instead of Very High) in MadVR to prevent dropping frames. It looked great and I was pleased but after a while I noticed the smell of toasty dust and a warm heater at my feet. Turns out even with these settings the RTX3090 was pretty well maxed out @ 95% ish GPU and a GPU Temp hitting 80c+. Fan noise is noticeable at a metre but it is not too bad and I don't think you would hear it watching a movie in a HTPC with the vol at normal levels.
Conclusion: Absolutely ridiculous for HTPC Playback for all but HFR HDR UHD playback. The only real advantage of the GTX3090 over the GTX3080 is extra VRAM (24GB vs 10GB) but this extra RAM is not required for Video Playback in MC RO HQ. When availability and prices settle down, I'm going to look at replacing my GTX1660TI with something like the RTX3060TI which I think will be a value proposition for HTPC playback in the new generation range.