Well they have low compression ratio pistons at 8.x(can't remember exactly):1 vs the standard zetec IIRC @ 9.6:1.
Also, these are the items that are different from the standard zetec.
1. The head had has different valves and flows better than standard
2. The water pump is a higher flowing pump
3. Engine internals can handle ~375-400HP
4. Intake manifold is a reworked version of the early 2000's plastic one
I am sure there are more differences but this is all I can remember off the top of my head.
Plus if you are going for an exact RS replica it is just another part of the puzzle.