Just think about that...the xbox would have to emulate 4 processors (r5900, vu0, vu1, IOP)
Not to mention most of the ps2 math instructions cannot be directly done on the xbox. (r5900 uses a 64-bit alu...)
SIMD could be useful for emulating the vector units, but that isn't going to help much.
Wait for the xbox2 or xbox3
To put it in simpler terms, how fast can the xbox emulate the psx do you think? That's the IOP...
It'd then need to be able to emulate on top of that, a 64-bit chip running only 2.5 times slower than the xbox.
It just doesn't make sense.