I have discussed this issue with a minor authority, and tempered it with what resonates true with me. First, let me say that depending on the musculature of your upper pectoral muscles and the flexibility of your upper back muscles, bringing your elbows closer together can negatively effect your breathing. You can test this while sitting in a chair if you lean forward and rest your elbows on your knees while breathing in deeply. You may very well notice that as you breathe in deeply, your elbows push outward spreading your knees. Albeit, we are talking about very narrow elbow spacing here, but the lesson learned is still to some degree applicable even with the difference between 38 cm width and 42 cm width (the current standard). If the lats are a bit stiff the effect will be more pronounced, and if the pectorals are large, to some degree they can push against the inside of the arms also and encounter resistance.
Now, here is the bigger issue to consider in relation to breathing and body posture. The biggest influence on how deeply the rib cage can expand and how deeply the diaphragm can descend is if the back is rounded and not flat. If the sternum drops down, the diaphragm can not descend straight down and the front of the rib cage is not free to expand. This is why so many authorities including Mr Carmichael stress flattening the back so strongly. Lengthening the spine and reaching forward with your entire chest cavity gives all the ribs the maximum room for expansion. A strong lower back and good core strength help to support the ribs so that they can expand in a relaxed manner.
I would conclude that it may be wise to use the narrowest bars that one is comfortable using to obtain maximum expansion without the power robbing problems of being un-aerodynamic. It is also a good idea to use the longest stem one can use and still be able to raise the upper body from the drops without jerking or gaining assistance from the arms pushing against the bars. This ensures that the sternum and chest have room to expand and that the back can lengthen. For most people the best position in the drops is achieved when the elbows are bent 45 degrees and the elbow is directly under the shoulder joint. As with anything concerning the body, individual bodies may vary greatly, and changes in position should be made gradually. A recent trend is to lower the stem many inches below the seat and this can either cause the knees to jamb into the rib cage (especially when using long cranks 172.5 and up). Another bad effect is what we are talking about here. In order to keep the knees from hitting the chest the cyclist will round the back restricting breathing, or straighten the arms, placing the body right back where it would have been had the stem been placed higher to start with. The old rule placing the tops of the bars an inch or two below the saddle and not four inches as is the current fashion, still applies. You can see the fashion gone crazy in all the Ebay ads that show the saddle nearly a foot above the virtual top tube (or top tube) and the stem all the way down making an apparent eight inch difference. I contend that no one can ride well like this and it just makes the bikes look sexier to those that don't have much of a hold on physical reality nor anatomy.
Garth Libre in Miami Fl.