Phoronix checks out the gaming performance of FreeBSD, you can find the results over here.
"FreeBSD provides a Linux binary compatibility layer that
allows 32-bit Linux binaries to be natively executed on this BSD
operating system. Linux binary compatibility on FreeBSD allows
Linux-only applications to be executed in a near seamless manner on this
alternative platform, even for games. New tests have revealed that the
modern FreeBSD operating system (via PC-BSD 8.2) can actually outperform
Linux when it comes to running OpenGL Linux game binaries."