That's not entirely true.
In some individual tests in the benchmarks on Phoronix.com, the latest open source ATI drivers reach now 80-90% of the performance of the closed source drivers (most are still at something like 30%).
Maybe 2 years ago, the best individual test results were something like 30% of the performance of the closed source drivers. Benchmarks that would not run at all on the open source side were a lot more common that today (although Phoronix may since have settled on tests that are known to run on open source, so take this with a grain of salt).
So on ATI cards the open source drivers have come closer, but they still have a way to go.
In the case of the noveau driver for nVidia, I find it impressive that the developers got it to run by reverse engineering at all. Performance, however, looks like that of the open source ATI driver 1-2 years ago.