Native FP24? Why not go all the way and support 4bit color, or grayscale even?
There is no way they are going to regress back to a more primitive core at this point. ATi will be moving to FP32 at some point in time, they will have no choice in the matter.
native FP-24 precision on the FX design would yield massive performance increases if they changed NOTHING else.
No, it wouldn't. The NV3X's design does not perform as we have seen due to the fact that it uses FP32 alone, it was other design decissions combined with that choice that ended it up where it is. Running FP24 all else being equal the NV3X would perform just like it does now in FP32 except with lower quality.
but I think 100% increases is a little optimistic
It doesn't require much optimisim to see 100% increase in pixel shader performance as completely viable for the NV40(v NV30). The big advantage using PF32 over any other current standard, they can combine the functionality of the Vertex and Pixel shaders in to one large shared shader unit. Besides that, they will certainly rectify the register limitations that made themselves so apparent on the NV3X core. Those factors alone could easily see PixelShader performance up ~100%, but the NV40 is a new core so it is far from unthinkable that they will also add more shader units anyway. How much of that will transfer to games is another matter, we need to start seeing some more shader heavy games before we can really speak much on that aspect(as it stands now we have two, TRAoD and Halo).