Windows 10 Radeon Software vs. Ubuntu 17.04 + Linux 4.12 + Mesa 17.2-dev
First up are some of the automated tests via the Phoronix Test Suite on Windows and Linux followed by some extra Steam gaming tests.
The Xonotic open-source first person results vary from Ubuntu 17.04 being a little bit slower to around the same speed as Windows 10 Pro with the latest Radeon Software driver to now with Linux 4.12 + Mesa 17.2-dev being noticeably faster than stock Ubuntu Zesty Zapus. The Xonotic OpenGL performance is now measurably faster on Linux than Windows.
With the synthetic OpenGL GpuTest benchmark on Windows and Linux, the performance largely sides with the modern open-source RadeonSI-based Linux driver stack over the Radeon Software driver. This isn't too surprising with the latest open-source Radeon driver stack generally being faster than AMDGPU-PRO, which relies upon the closed-source OpenGL driver that is shared by the proprietary Windows and Linux AMD drivers.