Rich from what I understand it happens to both pre and face lift cars and there's only been a very small percentage that have actually split liners . I think it's more to the fact of bad tuning running to much pressure combined with a lean afr . I stripped a focus rs mk2 engine for a mate the only difference between the st and rs from what I found is the most important compression is 8.5.1 on the rs ,st is 9.0.1 and the inlet cam on the rs is 0.8 mm more lift , now obviously the rs has the less gap iirc 5.6 mm rather that the early block of 14.5 mm but that is the only difference