(Potentially relevant link)

The 1913 edition of Webster's Unabridged Dictionary has this to say about 'Sufficience':

Suf*fi"cience (?), n.

Sufficiently.
[Obs.]

And this on 'Sufficiently':

Suf*fi"cient*ly, adv.

To a sufficient degree; to a degree that answers the purpose, or gives content; enough; as, we are sufficiently supplied with food; a man sufficiently qualified for the discharge of his official duties.

Last I checked, nouns and adjectives were rarely synonyms.