From what I've heard/read, RMS does not normally cause an engine failure, because you can catch it if you notice the oil leak under the engine and have the seal replaced for around $800. I think 20% is a fair number for the people that have an RMS leak, but I think the % of people that have to replace their engine is much lower.
IMS normally occurs without warning and suddenly but IMS only affects Boxster's built from 2001 and on, because they have the "better" redesigned IMS.
The pre-2001 Boxster's very, very rarely have IMS failures. The only major worry about pre-2001's is a slipped sleeve issue from a 10/98-03/99.
This what I now about what can cause an engine failure, I'll let everyone else try and figure out what the percentages are.
