Is it possible that you're overlooking something? A few things I believe.
Primarily, any large jump in clock accompanied by an INCREASE in per cycle efficiency is a great accomplishment. The fact that the g5 is only marginally faster per mhz should be impressive to you, not under-satisfactory.
Secondly, Apple has succeeded where AMD has failed. Hailing 64-bit as the end all for Intel. I never cared enough to check, but from what people have told me the Opteron has had it's ass kicked in places where it needed to shine to become a desktop competitor. While I'm sure that in the server world where there exists a market for highly per-system optomized code and software, competing in the desktop space is far more difficult.
Third, Apple hasn't yet finished a fully 64-bit version of MacOS X. Many computations in OSX are (thankfully) passed on to code created by Apple. This is how I believe Apple has managed to squeeze out so much performance, specifically through use of altivec acceleation in system frameworks, compiler, etc. In the future when we have a full 64-bit compiled AND OPTIMIZED version of Panther we can expect even more superior performance.
Fourth, you can argue Apple's tasks are biased. If you play WWDC again you will notice the P4 does well and dandy until it gets to scaling down this massive TIFF of a whale. If I remember right, 64-bit chips should do best in massive computations of that type. It's clear that given the task of optimizing Photoshop for the g5 in order of what is used most and where the best performance would come from, scaling would be high in that list. But like the infamous debates over Quake3 driver tweaks, technically (I believe) that optomizing for something like scaling in phohshop then that task being the difference in winer and loser in apple's P4 vs G5 showdown is a fair fight. Who's to say that that isn't what has been killing performace on large photoshop projects all this time?
I don't know if there is a way to write for 64-bit specifically, or if all the 64-bit tweaks are done in the compiler. But if there is a way to code with 64-bit in mind that will be another example of increased performance to look forward to in the future. In my opinion it's not a question of if this current g5 line up will beat the new P4, it will. But I'm afraid Apple will no longer win on SPEC tests unless some crazy stuff happens inside the new P4s. Still If you remember from the days of "slow g4s" Apple was forced to very heavily optimize their code, as did 3rd parties. Now since most of that optimization took the form of Altivec we can reap the benefits on the g5 immediately, due to it's highly advanced Altivec support.
Outstanding