The point of Smart Access Memory is so that applications can fully address the entire VRAM buffer, it's precisely what it does. I understand it does not affect Infinity Fabric frequency (in fact I never said that), but adding faster memory means you can feed the CPU faster and AMD has shown the gains themselves. Higher VRAM bandwith from GDDR6X should improve on these gains.
As for the RX6000 the on-die cache is a GPU cache and SAM memory operations are handled by the CPU. The GPU cache cannot cache operations that were not processed by it. Notice how AMD never mentions the cache and repeatedly talks about the "high bandwitdh GDDR6 memory" when talking about SAM. For reference: