Graig A Hunter's Paper
In this phase of testing, an existing version of the NASA Jet3D [1] code (developed and compiled on G4 systems) was run on a G5 system without any re-compilation or additional optimization.
As I read this paper, there isn't any significant performance gain by using a non-optimized version of their code. All this paper really states is that the G5 runs the code on par, or slightly better than it did on a G4 or a dual Pent. running RedHat.
OSX and the G5 are pretty intertwined. It will be interesting to see benchmarks of optimized code on several different kinds of machines down the line.
I don't know if this is the best review of the G5's preformance.