in so far vs to that extent

in so far and to that extent both are adverbs.

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

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

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

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