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.