Mmhmm, I think there is a difference with the way FPs and FJs show their feelings. I'm not a believer in the cognitive function theory (i.e. that there are introverted or extroverted variants of the functions), but I do believe that perceiving/judging has an effect on the way one expresses F. The big difference, I think, is that I notice FPs observing people much more often than intervening to change their emotional state, whereas I have an uncontrollable desire to change "sad" to "happy" in others. You know? It's a very quick reaction. My observation of INFPs and ENFPs in comparison to myself is that they are seemingly more "detached," not because they don't feel sympathy but because they don't have that instinctual need to intervene quite as often.
Hmm, I'm not sure if you've looked into the socionics quadra, but here's an interesting link:
http://www.socionics.us/theory/quadras.shtml
I have this suspicion that I'm actually INFJ, even though I can be chatty and interested in others. If that's true, then we'd both be "delta quadra" types, and it says that delta quadra types like discussing internal motivation and self-development.