I can tell you my experience with a 9900k to a 12900ks upgrade some months ago, keeping my 3080Ti card on a completely new PC including also M2 disks and DDR5 RAM. The performance was nearly identical even if the CPU, MoBo and HDD + RAM were faster. I maybe gained 5 fps average. However the game loading times and camera panning were improved massively.
Game currently relies on two principles:
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It runs a main thread which is blottlenecked by design and only Asobo can fix it (basically many things run on that thread). A faster CPU can help but it´s not the real solution as there are situations where relevant freezes lasting up to 1 second still happen on it (they started with later SU10 betas and I think they are caused by the ongoing memory optimizations). More CPU cores won´t help either as that makes CPU run cooler basically, because there´s still only one main thread.
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It puts a hell of data on the VRAM. Even for the 3080Ti (12 GB) that can be an issue and it can be close to 100% VRAM usage so a card with more VRAM helps. The alternative solution is to reduce textures resolution to high or TLOD to 200. When complex scenes are rendered a fast GPU clock helps but the one of 3080Ti card still serves for that purpose quite well but lacks some more VRAM to support it.
Cheers