There are alot of arguments that can be raised with this article in mind and this speed tweak.
The first thing to keep in mind, although the X800 can be tweaks *specifically* for Doom3 using this Hardware Calculation; Carmack himself noted that although there *might* be a performance increase, he didn't believe it would as large as being claimed.
Another thing to note is he also said that the entire shader engine relies on these lookup tables, which means if you replace these there is a change the end result will not appear to run the same.
There is also the fact that using the 6-Series new architecture, it is likely that the 6800 Cards will see the exact same performance boost from doing it hardware.
Something interesting to note about Doom3 though, is that when we have the X800 XT (575/580/16pipe) against the 6800 (320/500/12pipe) we are still seeing the 6800 in the TimeDemo which is a pure graphics benchmark outperforming it by around 5-10%.
To add further insult to injury, in standard gameplay your looking the FPS being affected because the CPU is being used more and what is aparent from this is that the Radeons are still very CPU dependant. What we see is thier FPS dropping even more.
We see this more on the previous generation of cards when running older processors.
http://anandtech.com/cpuchipsets/showdoc.aspx?i=2149&p=1
A Combination of an Athlon64 3000+ w/Radeon 9800pro will very noticeably out perform your XP2500+ w/Radeon X800pro
