I don't know where you heard R300 can do FP16, I have never heard that stated, and it does not seem likely. people "in the know" always say that R300 does not support FP16, and does not support true 128bit color either(FP32). we all know it's max is FP24.
i'm not really sure where you heard the R200 pathway uses 16bit floating point precision, AFAIK Carmack has never explicitly stated such a thing
Where I got that was from mis representation of its specs from ATI,
Then again ATI specifically states it supports 128 bit precision too
According to their specifications its support 64 bit floating frame buffers, But thats another things,
Since aparently its just 24 bit downsampling to 16 bit (which I think is retarded for any given number of reasons)
And ya, Carmack does state the r200 uses 16 bit precision in Its shading, It has too. To support r200 features. So therefore, The R300 is aparently down sampling its quality, Which isn't exactly a good representation of performance.
Actually carmack openly supports 16 bit precision over 24 bit and 32 bit. This can be seen in his beyond3d interview
IMO being strictly 24 bit is ATis Gift and its curse, Because they won't benefit from good programming such as carmacks precision modifiers in 16 bit, As 24 bit isn't really offering any IQ over 16 bit.
But they also have the benefit of being "just enough" For specifications,
Either or, I think ATIS implementation of its floating point precision kinda leaves a little bit to be desired. Expecially when you consider DirectX 9.0 current specification. As ATis card is just a bare minimum for DX 9.0 I'm not quite sure they chose to stick with strict 24 bit precision. DX 9.0 specifications be damned. Probably to save Die space on their already crazily overloaded 0.15 micron proccess.
From a programmers point of view, They leave little room for modification or tweaking, And thats always a bad thing, I can see why John Carmack Stated he has become limited by the r300 programmability. Kinda disapointing to me. Oh well.