These numbers don't seem too awful. In Cinebench rendering, the G5 only misses being a "Mhz scaled G4", by a few percentage points, which is what we've been told to expect from 10.2.7. We know the G4 had a better graphics card in it and I don't believe it said if they had the same RAM, unfortunately. I suspect the G5 would have done better with more RAM itself.
Now that I think about it, I seem to remember that Cinebench is currently working on G5 updates. (Please, correct me if I'm wrong.) If these are non-optimized numbers we're seeing, I say it paints the G5 in an excellent light! The architectures are vastly different and if it's doing this well with G4 code, it's going to look good with G5 code.
Baz