My shoulders are actually 38, but my bike is a men's frame and 40 was the smallest handlebars available. My previous bike had 38...I can't really say that I notice a difference, but that's mainly because my 2 bikes handle so differently to begin with. My reach on my previous bike was really a bit too short for me. Current bike has a slightly longer top tube and a 20mm longer stem (started out with the shorter stem, but handling was scary and I felt cramped).

I can say that I wouldn't want my bars any wider, though. If they were wider I don't think I'd be comfortable riding on the hoods.