I don't know for other vendors, but at least for nvidia, there's parity with the Windows drivers, according to the benchmarks and the general stability. In fact, I remember when sometimes a feature had to be removed from the Linux version of the nvidia drivers just to keep "parity with the Windows one".

IIRC most of the nVidia codebase is platform-agnostic with small bits that shim to the underlying OS, rather than developing two entirely separate codebases.

And yet I use mine to play video games all the time with zero performance loss that I can tell.