good question. NFs tend to score low on E/I. especially as they get older. the idealist temperaments share a number of introverted talents (introspection, intrapersonal intelligence, independent thinking, vivid imagination, creative, free spirited nature, philosophical bent) as well as a number of extraverted talents (public speaking, interpersonal intelligence and relating to people, personal problem solving). I'm an ENFP and I score very low on E because I take a lot of time to sort things out in my brain (Ne works by gathering information, making connections, and then getting really excited and wanting to tell everyone, this gets annoying at school because I feel the urge to tell all the connections I make that the teacher does not address, even in class that I think are boring is hell. it's almost like an instinct or something and I have to be careful to keep my mouth shut or I can talk for like 10 hours straight).... anyway I digress, introversion/extraversion does tend to decrease with age, but more so for NFs than other types (the time when mbti type is most obvious is in the teen years when the 3rd and 4th functions are fully developed)
an INFJ with a strong Fe (for instance, a revolutionary leader like Martin Luther King) is probably going to score very low on introversion. likewise, an ENFJ who is an author will probably score low extroversion.
I think the main difference is that ENFJs will tend to get more excited. they love to be constantly talking and connecting with people where an INFJ will be more serious and up in their head, having a few VERY close friends (this is common NF trait though, we tend to be extremely picky about our friends and would rather have 2-5 close friends that we can really bond with). INFJs tend to fall into the tragic hero role and feel like they have to put everything on their shoulders and be Prometheus for everyone (this kind of annoys me, ISFJs and ISTJs do it too and ENFJs do it a little bit)