Phoronix compares the performance of Ubuntu 11.10 and Mac OS X 10.7.2, you can check it out over here.
After delivering benchmarks last week that were comparing the
Intel Sandy Bridge performance of Mac OS X 10.7 "Lion" vs. Ubuntu 11.10
"Oneiric Ocelot" when it came to the Sandy Bridge OpenGL graphics
performance, here's a comparative look at the performance of Ubuntu
11.10 against Mac OS X 10.7.2 from the Intel Sandy Bridge-based Mac.