In English, good is generally an adjective while well is generally an adverb. In this case the good/well is referring to "it" as "it is not enough." Since "it" is a noun, you would use "good" (the adjective).
I think no, well would indicate that you are thinking and need some time to say something.
"Fine, but it is not enough" is also a valid translation for this sentence.
It seems like abbastanza is used as an adjective and sufficiente is used as a noun, ma non sono sicuro.
Sounds like post much commentary on Nadal's opponents after defeat:
Good but it is not enough.