For the OpenMP-based Fortran-written NAS Parallel Benchmarks from NASA, Debian GNU/Linux tended to be significantly faster than Debian GNU/KFreeBSD.
Debian GNU/kFreeBSD had a slight edge for VP8 video encoding, but Debian GNU/Linux had a striking lead for x264 video encoding.
While Debian GNU/kFreeBSD is quite an interesting Debian port, at this time with the FreeBSD 9.0 kernel it generally was slower than Debian GNU/Linux with its current Linux 3.2 kernel. However, for many of the tests the results were close with the similar user-space.
Now if only Debian GNU/Hurd was as compelling...