You are basically correct. In the examples you give (as far as / insofar as I know) there is no real difference except that, as you say, "insofar as" is very formal and in my opinion is better avoided. There are times when you must use "insofar as" and "as far as" won't work. Merriam-Webster gives this example: "The news is good insofar as it suggests that a solution may be possible." "As far as" won't work in this sentence. What "insofar as" implies in this sentence is that actually the news is not really that good at all, but at least there is one good aspect of the news, which is that it suggests a solution is possible.