The Qfusion-powered Warsow game performed close to the same under both operating systems with the NVIDIA GeForce GTX 600 series Kepler graphics card.
The Xonotic graphics performance results varied a bit between platforms, but overall it was quite tight between the two operating systems with the largely shared driver code-base.
Overall, these results are not too surprising. For the most part the NVIDIA GeForce GTX 680 performance between the two operating systems was more or less the same, which is to be expected when the driver code-base is largely shared on NVIDIA's end between Windows and Linux (and BSD and Solaris). For some OpenGL games, there were measurable differences, but nothing that would make or break your ability to run the game on the GK104 Kepler graphics card.