You have got to be kidding.
jeffrey, does anyone really claim that an angled shoulder seam increases elasticity? What I have read, and had explained to me by tailors, is that the angle (caused by the small backneck measurement) moves the neckpoint back and down in such a way is to increase the collar's ability to stay on the neck. Also, it allows for extra fullness over the blades, which helps the coat move more freely.
Both of these observations are true to my experience. Every coat on which I have had problems with the collar not sticking to the neck have been ones with straighter shoulder seams. All of the coats that I have had with angled seams (and small backnecks) -- whether from London, Italy, or New York -- have stayed glued to the neck at all times.