Not sure if that'll still be the case next year though. AMD is pushing Linux hard, whereas Nvidia is being actively hostile to things like VFIO (the code 43 error) because they want you to buy a Quadro for no other reason than to line their own pockets.I recall reading some time ago that the Linux port of Left4Dead was outperforming the Windows version on the same hardware.
The latest comparisons I've seen show Nvidia's Linux drivers still outperforming AMD's.
There are some outliers for games, for sure. I don't recall L4D because it's so old, but take for example games that use OpenGL instead of DirectX (like Source titles like L4D). AMD's OpenGL implementation is significantly better on MESA in Linux than on Windows (or the propriety AMD-GPU driver on Linux which uses the same code base as the Windows version). A recent example, the CEMU emulator for Wii U, runs better and more consistently under Wine with the open-source MESA driver in Linux than natively on Windows.
Another example I remember is that at 1080p, for BioShock infinite, Windows runs better. But at 4K or higher, the Linux port runs better and more consistently.