<font face="Verdana, Arial" size="2">Also, doing the form this slow has really made me concentrate on balance and transitions.</font>
My personal theory is that "concentrating on balance and transitions" becomes an urgent requirement as one improves. I think it is critical in order to begin deepening understanding of Jin and how it flows.
<font face="Verdana, Arial" size="2">I've found, perhaps ironically, that I experience less breathlessness the less I try to breathe externally, gulping air into the lungs, than internally, from the abdomen, making the breathing 'fine, slow, long and profound'. ... It's difficult to slow it down this much from those points of view, at least if you're thinking about the ten essential points every centimetre of the way, as well as the breathing and applications. </font>
My own view is that concentrating on breathing can, paradoxically, decrease the development of internal aspects of Taijiquan. For me, "making the breathing 'fine, slow, long and profound'" has less to do with breathing per se than with a certain type of movement and attitude towards movement.
If you "relax" and calm your mind and spirit, then your breath will naturally tend towards "fine, slow, long and profound." In this way, your breath will be an indicator of how smoothly you are circulating Qi and Jin.
If you control your breath to make it "fine, slow, long and profound" independently of your other movement, I think this will not provide the best result. I think a good analogy can be made with the hands.
It is often said that the hands can express the entire quality of one's movement. Despite this, if you merely focuses on making hand movement even, coordinated, and correct, this would not result in very good Taijiquan.
I attended a few seminars this season where Yang Jun put increasing emphasis on "internal" aspects of doing the form. While he did mention aspects of "natural" breathing that should be expected, he seemed to put more emphasis on feeling the flow of Jin like waves. No sooner does one "wave" crash and expend its force than another begins to rise to take its place.
For me, critical elements of having this feeling are keeping my mind, body, and spirit calm and unhurried. Another critical element is concentrating on the "meaning" ("YI") of each posture and transition and how it relates to my current body positioning.
I find that as I begin to do the form slower and slower, I begin to lose the feeling of waves. The oscillation becomes too tenuous or too subtle for me to deepen the feeling. It feels more like floating in a river, than bobbing in sea swells. I am not sure how slow is too slow, but this is one of the feelings I use as a guide. Doing the form faster and faster also causes problems.
Of course, the difference between inhalations and exhalations would seem to provide a way of connecting on the feeling of waves. I think there is something to this idea; however, I also think that concentrating on the breath would be to do things in reverse. Your breath should follow your intent, rather than the other way around. I also think that the meaning of the postures is too detailed to match any particular breathing pattern. There are waves within waves and patterns within patterns.
I hope this helps.