Greetings from the finally thawed and slightly dried NE,
I'm not going to present a logical argument (or an illogical one) either for or against wide vs. narrow bars relative to breathing efficiency, but will give some personal experience - of course, as Mr. H. Sachs says: Your Mileage May Vary........
I prefer narrower bars, i.e., <40cm - wider bars cause me to feel a bit "disconnected" from the bicycle's "centerline" - I am 5'10" and ~168# which is just about average (I suspect?) - I have been riding derailleur bicycles since 1969 (an Atala) and have ridden motorcycles aplenty as well. I also ride MTBs, and HATE the wide bars (I usually cut 1" off of each side).
I have not noticed any less (or greater) ability to breathe relative to bar width. As I wrote above, wider bars cause me to feel less "connected" (especially when cornering) than do narrower bars, but I have not experienced any noticeable change in breathing ability.
Hopefully, things will dry out soon so I can go for a "first" ride on my n ewly completed Serotta nhx with 39cm Cinelli Campione de Mondos!!
Everyone have an enjoyable Holiday weekend.......
Chuck Brooks
Malta, NY