It depends what you mean under 'performance'. Benchmarks like geekbench largely measure isolated aspects of CPU/RAM performance and should be unaffected by the OS (unless the OS really messes things up). They can be used to measure the memory management subsystem by the OS (at least partially). Now, Xbench — thats a truly pointless benchmark IMO, especially their UI benchmarking makes no sense to me. UI refreshes are generally locked to 60fps, so I am highly sceptical about how it does its comparisons and what results it produces.
So, again, what kind of performance are you after? Memory allocation? Disk performance? network performance? OpenGL acceleration?