Some of these tests are clearly testing the compiler more than the OS. Looking over the scimark tests, for example, the FFT, LU, and SOR numbers are (at least)

Some of these tests are clearly testing the compiler more than the OS. Looking over the scimark tests, for example, the FFT, LU, and SOR numbers are (at least) an order of magnitude too low to be using optimized libraries that are part of the system. For these benchmarks, we're seeing the compiler quality of compiler codegen on a reference C implementation (OS tasks like malloc/free efficiency may have a small effect, but it should be essentially negligible). No one interested in performance would actually use that code on OS X or Ubuntu; you would use a tuned library.