I've had sp/so in tests some of the time, or even so-first. I think this has to do with strong Fe and societal demands, though. I identify much more with sp/sx descriptions, especially in relation to type 6. But I am also sure I'm not sx-first.
I'd say I identify as sp/sx in part because of the following:
-I can be very passionate and even obsessive about others, especially in a romantic context (slightly stalker-ish tendencies), but they tend not to know as I maintain a cool/laid-back exterior (a bit too much so, apparently).
-@skylights said something a while back about INFJs being "formal but warm" (or words very close to that), which really resonated with me. I think it fits well with a lot of INFJs but it fits me particularly well as an INFJ type 6 sp/sx. I think some people really warm up to me for that very reason, and others don't know what to make of me. I think that sx/sp would be a more obviously conflicted type and more obviously emotionally volatile - I have a friend who I am pretty sure is xNFP type 4 sx/sp, and that really fits her. I am more conflicted than most people would think, but it is not very obvious externally. I do think I give off a sort of...conflicted undercover vibe (also reflected in looking externally calm, but being very anxious, sometimes to the point of neuroticism) which some people pick up on and either accept and want to explore, or they are put off.
-I appear self-sufficient, and in some respects I am, but I worry a good deal about the health of my relationships with others. I rely on friends to an extent they probably don't realise. I am also more concerned than most people would know about romantic relationships and whether others find me attractive (both in terms of personality, and physically).
-One reason I think I'm so-last is because although I take a lot of my self-worth from the health of my relationships with others, it tends to be on an individual/one on one basis. I've met many people who are very concerned with social hierarchy, ie. getting in with "the cool people", achieving status, and so on, and while it can sometimes trouble me, I have more contempt for it than anything else. My reaction is along the lines of "grow up - you're not in high school any more", though I may also worry about being excluded. But I worry far more about losing a relationship (whether friendship, or romantic) with someone i care deeply about, than I worry about fitting in with a group or in some kind of hierarchy.