Not really, none of the Vulkan drivers in Mesa are built on top of Gallium.

Yes, it works the other way: Zink is the Gallium->Vulkan translation layer, while the main Mesa code is effectively an OpenGL->Gallium translation layer.

This is the whole point of Gallium, right?

Like, the classic "Intel OpenGL driver" in Mesa (i.e., i965) doesn't use Gallium and NIR, and hence has to implement each graphics API itself, whereas their modern "Iris" driver using Gallium presumably just handles NIR -> hardware?

Or does the Gallium approach still require some knowledge of higher-level constructs and some knowledge of things above NIR?

Does that imply a performance hit, or is it roughly equivalent to targeting Vulkan “directly”?

Think of it as HAL, on top of which state trackers implement their chosen APIs. OpenGL is one of them, there's also Gallium Nine that implement DirectX 9.