I poked around a bit at VRAM speeds when I was doing the Sapphire review. Ultimately, I didn't say much about it, but even though you can push clocks higher, I don't think you get the gains that I'd expect. There's something goofy with the VRAM speeds on these 18Gbps modules where you often don't get anywhere near the theoretical boost in performance relative to the existing 16Gbps cards. I suspect memory timings (which you can't directly see on the GDDR6) are somehow at play.
For example, and I know this is a specific use case, but the cryptocurrency mining speed of the RX 6950 XT was consistently far lower than the RX 6900 XT, regardless of what I tried. You can get ~65 MH/s out of the RX 6900 XT after tuning, but the best I ever managed on the RX 6950 XT was about 54 MH/s. "Stock" (factory) performance with a tweak to the maximum GPU clock did better than any attempted memory overclock.