Not in D3D11 itself, but due to it being the prevalent API, drivers are optimized for it. See https://medium.com/layerth/benchmarks-dota-2-streaming-feat-... for example.
> FYI, "bytecode shaders" have nothing to do with shader performance. They have some impact on initial parse time, but little else.
You're right, it shouldn't make that much of a difference, but I think driver/compiler optimizations for HLSL (and lack thereof for GLSL) is one of the reasons, games are faster on Windows using the AMD driver than Mesa. It also plays a role that most ports are using HLSL to GLSL translators.
There definitely are cases where the open source driver outperforms the proprietary Linux driver. But I haven't seen a case where a game runs faster on Linux than on Windows with AMD hardware.
This statement utterly untrue, there is nothing especially performant about D3D11. FYI, "bytecode shaders" have nothing to do with shader performance. They have some impact on initial parse time, but little else.