so far vs to that degree

so far and to that degree both are adverbs.

Word	Noun	Adjective	Verb	Adverb
so far	No	No	No	Yes
to that degree	No	No	No	Yes

As adverbs, so far and to that degree are synonyms defined as:

Other synonyms of so far include in so far, insofar, to that extent.

so far (adverb)	to that degree (adverb)
used in negative statement to describe a situation that has existed up to this point or up to the present time	to the degree or extent that
used after a superlative
to the degree or extent that