Also, a while ago Michael showed that enabling PCIE-2 support gave quite a boost to the radeon cards. I'm guessing nouveau drivers have that enabled.
I'm pretty sure it's because they are less CPU limited. (or PCIE bus bandwidth or latency limited, which amounts to the same thing)
The binary drivers have gone to great lengths to make sure the GPU is never stalling out waiting on the CPU for instructions or data. The mesa drivers aren't as good at that, but for slower cards often the CPU can keep up reasonably well. With the faster cards it becomes really obvious that some of those optimizations are missing.
With the nouveau driver coming quite close to being on a par with the closed blob, nVidia should embrace this so at least people can continue to use their older cards that are no longer supported by their blob or even provide an alternative.