I did actually mention it in the first paragraph of my original post

for exactly the reasons you mention. There are way too many differences between platforms and game code for any of the FPS figures to be taken literally. The figures were to illustrate the relative performances of various GPUs.
Yes, I did notice the downclocking of the Radeon 9600 Pros. If you check some of my posts from earlier this week, I've been messing around with the ATIcceleratorII overclocking utility, and sure enough, the default clock speeds of my G5's Radeon 9600 Pro are well below standard. As you say, it would not be at all surprising to see a similar downclocking used in the iMac G5, but I hope against hope that this is not the case.
Yes, the smart buyer will definitely be waiting a few weeks to see how the benchmarks turn out. At this stage it is unknown what effect a) reduced G5 FSB bus speeds b) single channel DDR RAM and c) the nVidia FX 5200 Ultra will have on the overall system performance of the iMac G5. My opinion is that the FSB and RAM changes will not have all that great an impact, but the FX 5200 is going to drag things down.
I've mentioned this in some other posts, but I'd love for the iMac G5 to be a good overall performer (including 3D graphics and games), so then I could replace my bulky 1.6GHz G5 PowerMac / 19" CRT setup with a svelte iMac G5. Unfortunately, I fear that the new iMac will be a little underwhelming in some key areas (most notably, 3D graphics and gaming).