to that extent vs in so far

to that extent and in so far both are adverbs.

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

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

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

to that extent (adverb)	in so far (adverb)
to the degree or extent that	to the degree or extent that