The 4x4 head is a true hemi head, and has better cooling properties. Apparently the cams that come stock also allow smoother running. Also it is tuned to work with US emission regulations (despite the fact the engine was NEVER offered in America)

The 2WD 2-piece exhaust manifold is MUCH better than the one on the 4x4. So many Cosworth 4x4 owners swap to the 2WD exhaust manifold for a cheap power boost.

The L8 ECU (4x4) is more advanced and is set to run to US emission specs. It is said to run smoother in a 2WD car as well. Better than the L6 (2WD) and the 3 door Cosworth (L1) in the fact that it offers more memory.

That's what I know. Anyone willing to fill in any blanks?