Well, DX10 was originally developed for the XP ME edition (good ol buggy longhorn) before that was gutted and they practically started over to make Vista. In light of that I can see how that's possible. Still, hacking XP like that doesn't always end well. Also, current drivers have alot of optimizations and such for various games to run better on their particular architecture, the main reason modern drivers are so large, so the performance probably isn't too comparable. It would be interesting to see some benches, but I wouldn't really bother installing it myself.
If you would read and do searches this has been talked about a lot before. It is not true dx10 it allows dx10 only games to be played on xp.
"These compatibility libraries allow the use of DirectX 10 games on platforms other than Windows Vista , and increase hardware compatibility even on Vista, by compiling Geometry Shaders down to native machine code for execution where hardware isn’t capable of running it. No reason for gamer to change or upgrade the OS and video card(s) to play the latest games."