to that extent vs so far

to that extent and so far both are adverbs.

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

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

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

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