> The 1,6 GHz PowerPC G5 seems to outperform the 1,8 GHz a bit
> (on the same machine, and the same memory sticks!): [...]
> OGR-NG: 21,027,973 nodes/sec
> RC5-72: 9,558,958 keys/sec

Beside the fact that these also seem off (but only by a factor of about 1.21) compared to the 2.7 GHz G5, your 1.6 GHz G5 being faster than your 1.8 GHz G5 indicates that something is not right here, which has nothing to do with optimization.