Most of my replies are personal and anecdotal, so it may be thought that I do not speak for all Fi users. Also, I can be close to the center INXP, and I use both. I believe you are right in that we use all the functions. I agree.
Perhaps when I'm in "Fi-dominant-mode", I'm caught up by feeling. Nothing may be on the surface, but inside I have feelings in my gut, in my muscles, and my whole being. It can be depressive feelings, anger, shame, or elation. It can be in relationship to thought or in response to internal and external imagery. These things are in the lead. The visceral is the factor of decision more than whether something makes sense. I like being led by good feelings, like harmony. In this case, thought may be involved in tandem or conjoined, but typically smaller.
In the harmonious stream...I'm seeking...like a fish seeking fresh water. I'm looking for a way to "fix something" with that person, so that I can be harmonious with them. Also, if there is something very negative (causing depression or anger) in relationship to a person, I'm trying to avoid it, and if I can't then I'll either blow up or have a break down. Of course, there's another option: LEAVE.
When I'm in "Ti-dominant-mode", it's more like a juicy or campfire sort of feeling, like Ummmm, this is interesting. It is more near the mind itself, and not in the body, gut, and whole being. This is what I may get if I'm playing chess, solving a math problem, or thinking about something. Feelings may be there, but they are suppressed or stifled. For example, I may have a feeling of shame while taking a math test (or definitely when I'm getting my ass beat at chess ), but if thought is interesting enough, I may have little feeling at all. Also, feeling doesn't help solve math problems -