The singular possessive form is ALWAYS 's, regardless of the final letter of the noun in question. Hence boss's, Jesus's, Keats's etc. Just because you see "Keats' poetry" it doesn't mean it's right; you see lots of grammatical and spelling inaccuracies in lots of places that should know better (my local hospital, in a large metropolitan borough, had dozens of signs pointing to the "Emergencey Department" for years!). "Goodness' sake" does, I agree, seem to reflect the way in which the words are usually spoken, but nonetheless it's grammatically wrong; and "conscience' sake" makes no sense at all - there is no use of an apostrophe without an s which could possibly indicate possession. Again, I concede that that's the way it usually sounds when people say it, but it's horribly ugly in print and would have got a ring round it from my red pen when I was teaching English for a living.........