The kernel driver is a rather small piece of the overall puzzle though, itps just the pipe that GPU commands are passed through. The bulk of the GPU driver code (and the majority of its impact on performance) is in the userspace components like the shader compiler and the OpenGL/Vulkan libraries. These are closed source.
The exception to this rule is that the kernel driver is responsible for power management and controls the GPU clocks, but as part of opening up the kernel driver NVIDIA made reclocking available for the fully open driver (nouveau/nvk) to use as well which means the performance differences between the two driver stacks are now down to optimizations.