I think it has the potential to be pretty good, especially on modern hardware. If using DXVK or VKD3D is any indication, I think the performance losses can be minimized. Plus, I've used Zink before on Mesa's RADV Vulkan driver, and it's pretty good.
Meanwhile, writing an OpenGL-on-Vulkan layer has been a plan for deprecating/simplifying driver support for OpenGL for quite a while now. Might as well focus efforts on that.
It's kind of like how Mesa's existing OpenGL drivers all switched to Gallium3D, which splits the Graphics driver into an agnostic front-end and a device-specific backend. Allows for a lot of re-use and optimization. I heard they're wanting to do something the same with their Vulkan drivers for the future as well.