i'm a solid NFP in keirsey... i'm so far to the N, F, and P sides that all of those seem pretty clear, even with the slightly different definitions. my Sensing is shitty and my Thinking is okay, but definitely not as good as, and also not preferred over, my Feeling. i'm also very go-with-the-flow. i suck at decisions so much, lol.
but as for I/E, i really don't know. part of me feels more Healer, and part more Champion. the Healer seems too selfless to me but the Champion seems too exuberant. i feel like i fall in between them. Championing Healer? Healing Champion...?
i do prefer function theory. it seems to account for the perspective differences more clearly than temperament. there's just more information to work with.