I find both concepts fascinating, but I've always had a few issues with the concept of passing. In many contexts, passing is contrasted with "being who you really are." But I have some healthy doubts about this concept that you could strip away all the layers of identity that your culture has imposed on you and find the true authentic you underneath it all. However, going down that road would seriously derail this post.
At the same time, I do see these identities (woman, black, gay, etc) as social constructions and scripts that are imposed on us/chosen by us/enacted by us/rejected by us... So I think the concept of passing can be particularly useful in conceptualizing socially constructed identities. When a "black" person (under the one-drop rule) masqueraded as a "white" person it was thought of as passing, because they were pretending to be something that deep down, underneath it all, they essentially were not. When a homosexual person maintains the facade of straightness, it's thought of passing because we believe that there are basic fundamental differences that undergird "gay" and "straight." And when a trans woman performs femininity it's thought of as passing because in our culture we still really tend to think that some mysterious biological essence determines your gender.
Beyond the essentialism (and problematic fake binaries) that the concept of passing reveals, there's the issue that the process of learning to perform a particular gender script and embody a particular social construction is much more visible to us when a person undertakes the project as an adult than when it is slowly learned, internalized, and perfected throughout our childhood. This is the norm, and it occurs in a largely invisible way.
And yet, if you really think about it, we're all passing. No matter how girly or burly you might be, nobody perfectly, completely, and comfortably inhabits every aspect of their assigned gender. Everyone has been exhorted to "act like a lady" or "keep your chin up" at some point, and most of us have figured out how to act the part and fly under the radar in many ways for the sake of convenience. And as the years pass and we become comfortable with the part we're acting, it begins to feel so natural that we forget that we had to make it fit at some point. So we think we really are masculine/feminine/straight/white/bi/whatever. Like deep down somewhere in our souls. This phenomenon is known as "bad faith" in the world of existentialism. I prefer to think of it as having sunk very deeply into the rabbithole and drunk from the bottle labeled "drink me."
So if we're all passing all the time to some extent or other, then it would seem that trans people and others who are undergoing a major transition are less likely to be in bad faith, because they're more likely to see gender as a script to be performed. But I doubt that this is the case. Among transgendered people I know (limited study population here), the commitment to the gender binary is no different than among the cisgendered population - they just think that they had the bad luck of being born into the wrong kind of body or assigned the wrong gender at birth. So in their case, passing is a more visible and more conscious phenomenon. It might occupy a lot of their time and energy, and be a success or fail proposition to them. But it's still thought of as a process of truly becoming feminine or masculine, and is not just a question of learning how to follow the script, and rejecting the script altogether is not an option.
And yet... it seems like we would all benefit by a major questioning of/stretching of/ditching altogether of the script. Why do we need these binaries anyway? I get the whole heteronormative "we need to know who's what so we can know who to reproduce with" shtick. But if you're already going to tolerate gay/bi/trans/whatever "lifestyles," then hasn't that ship sorta already sailed? I mean, if the original idea was to have some sort of organizing schema by which you could look at someone's outward appearance and know whether or not they were a potential mate for you, then clearly our schema has already become deeply dysfunctional, so why cling to the fundamental and antiquated elements of it?