So I found these benchmark results from somebody using a 6000 series card
https://www.tweaktown.com/articles/9691/amd-radeon-rx-6800-xt-in-mgpu-big-navi-gpus-insane-performan...
Especially in 4k there seem to be a dramatic advantage to mgpu performance
I recently purchased a Samsung Odyssey G9 neo which cannot be fully driven by any modern flagship GPU yet @ 4k+4k 240hz
The 4090 is limited to 120 hertz by the older DisplayPort standard
https://wccftech.com/nvidia-geforce-rtx-4090-gpu-samsung-odyssey-neo-g9-monitor-not-a-perfect-match-...
The 7900 XTX (which I recently ordered) has 2.1 DisplayPort standard so it has higher bandwidth to drive the screen at full resolution but still won't be able to reach maximum frame rates
I'm hoping to find someone who has benchmarked the RX 7900 XTX cards in mgpu configuration @4k in comparison to 4090 benchmarks before I commit to another $900 GPU
If you found any additional information since your search last October I'd be very interested