Bare Feats has posted an impressive Power Mac G5 Quad 2.5 GHz Cinebench score of 1101. The previous highest Mac score was held by a dual G5 2.7, and was less than 700. The 1101 score by the Quad G5 is about the same as one would get from a quad 2.4 GHz Opteron 280 machine with the 32-bit version of Cinebench. (The Opteron scores higher with the 64-bit version of Cinebench under 64-bit Windows, but there is no 64-bit Mac OS X version with which to compare.)
Although the single processor score was not posted in his review, Rob at Bare Feats was kind enough to tell me via email that it is 359, and thus the multiprocessor speedup is 3.07. This disproves the theory held by some that Cinebench was being held back by poor support of quad CPUs in Mac OS X. This idea was put forward when another quad G5 tested got scores of 359 and 1016, with a low multiprocessor speedup of 2.83. While the speedup of 2.83 is low, Bare Feats' speedup of 3.07 for the Quad 2.5 is quite good, and in line with what would be expected with quad machines based off dual-core Opterons and Xeons. The large variation for the multiprocessor score is curious however, although not unexpected since previous dual G5 Macs also had large variations in the multiprocessor scores.
In addition, the Quad G5 2.5 970MP has been tested by Luxology and it turns out that it is more than twice as fast as the Dual G5 2.5 970FX in modo. It seems the 970MP's increased L2 cache (for each core) really helps performance in this application. It is also interesting to watch their load balancing video which demonstrates that all four cores are at 100% utilization during the render.