There are several projects trying to bring dx10 to XP, but it's not easy or simple. There are functions and operations that dx10 has that dx9 does not, and these functions must be emulated. This emulation isn't simple, there are noticeable differences in how the API is designed and how the device interacts with the system as a whole. These differences must be overcome.
Even if a plugin runs better on dx10, it does so because there's hardware support for whatever the speed increases depend on. In XP this will be done on software, much slower.
An over-simplified example goes like this:
Say dx9 had functions to rotate an object along the X and Y axis, but dx10 has functions to rotate an object along all three axis (X,Y,Z).
You can accomplish a rotation on the Z axis using dx9, but it requires multiple rotations along the other axis. So, while you can technically implement the dx10 function on dx9 hardware, you do so at a cost. In this example it's obvious how to mimic dx10-functionality on dx9-hardware, but in the real world the dx10 functions have no obvious analog in dx9.