Before you make the argument that ATI is "cheating" you will have to show me the IQ differences between FP32 and FP24.
. There you can see FP16 on NV35, FP32 on NV35, and FP24 on R350. Whereas FP16 stands out, FP24 and FP32 look the same to me. The water level is different, that is the only change I see. I don't see any more graininess or banding in FP24 than in FP32, do you?
As a side note, are you really trying to say that FP24 is out of spec for DX9? That's utterly ridiculous. There have been discussion about what the minimum spec is, I'll look later. Regardless, this has been defined by Microsoft, I'm sure if you look for it, you will find where it says FP24 minimum.