Handlebar width is VERY individual. It isn't narrow=more aero in a linear scale.
I've got bars from 38 down to 27, and once I'm below 36 the amount of reach necessary to keep the elbows from flaring out quickly starts to increases to the point of failing a jig (and becoming unreasonable to ride) and I'm by no means a "large" guy.
I think most people will see a little benefit going from something like 40+ to ~36, and those that really hone the position (and focus on it specifically) perhaps down to 32-33. Much below that imo starts becoming the oddity that sees gains, and most see detriment.