so far vs to that extent

so far and to that extent both are adverbs.

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

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

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

so far (adverb)	to that extent (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