I don't like B either. "So" is best left out of this type of expression. I agree "isn't...enough" (to do/achieve some result) is the best construction; or "isn't sufficiently...".
But for me, "isn't so...as to (achieve some result) is not attractive; I'm going to stop short of saying it's wrong, not simply because Kevin has approved it but because I'm sure it is used in spoken English frequently enough.
I agree with El escoces about the usage of "so". I would certainly avoid construction B) even though it is probably not wrong. Kevin Beach's example (in bold) is how I would say it; this seems to be by far the most natural way.