to that extent vs insofar

to that extent and insofar both are adverbs.

Word	Noun	Adjective	Verb	Adverb
to that extent	No	No	No	Yes
insofar	No	No	No	Yes

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

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

to that extent (adverb)	insofar (adverb)
to the degree or extent that	to the degree or extent that