It's interesting that the eMac had a 167Mhz Bus, while the iBook G4 had a 133Mhz Bus, and both used PC2700 Ram.
As we'll recall the G4 never supported DDR, meaning we never got to take advantage of the Double Data Rate of the Ram. So the Ram just runs at the bus speed.
Finding the PLL that sets the Bus, we should be able to increase the performance quite a bit for tasks the are memory bandwidth bound, at least for the 133 Mhz bus models.
Also, Macbench isn't really the best benchmark, it was never pure carbon, so you don't know how much of the 68k emulator you are benchmarking vs. PPC code. Plus I don't think they ever updated it to take into account CPU Cache.
Real world benchmarks bear out the 7455 with it's L3 cache is much faster, even at a lower clock speed than any 7447 could be. In particular 3D gaming, however if the task can't take advantage of the L3 cache clock speed wins out, and the 7447 maybe faster clock for clock than the 7455.
http://www.xlr8yourmac.com/G4CARDS/powerlogix_dual_g4_7447_7457/index3G.html