I've no expertise in such, but I'd guess those patients in intensive care units (about 12% - 15% of Hong Kong SARS patients) would not be strong cndidates for survival.
I did see one figure given by a journalist in Hong Kong, but she gave no source for it, attributing it only to "doctors". The figure was 40%.
Agreed, that's a best-guess scenario as far as I'm concerned.