I have not had time to test it for performance since messing with it. It can be difficult to say though since kind of overhead cost can be very platform specific. Sometimes the overhead is merely from having more temporary variables in the overall shader file which even if many of them are not used (due to branch always being disabled etc), can cause the GPU to process things less efficiently. But sometimes that cost may only show up on ps4 for instance. In practice on PC with a decent video card it is probably hard to measure the cost since it will be very small. But when enabled, it makes all clearcoat materials go up in instruction count a bit. I can’t remember exactly how much but it wasn’t double or anything crazy. If you enable it in project settings but don’t use the bottom normal, it will still add some cost to the clear coat deferred shading since it has to check if the pixels encoded the bottom normal mask. For materials that do not use it, it will not really do that much extra work but the extra variables in the shader may cost something small.
GPUs try to parallelize things they will ‘make room’ for variables they don’t know they won’t need which is why unused extra variables can hurt in some cases.