StephenH

"As for aerodynamics, you body is wider than any handlebar bag"

I've heard this line of reasoning before. The problem is that "drag coefficient" is usually defined in terms of frontal area. But that doesn't mean that aerodynamic drag is ONLY determined by amount of frontal area. If you change the shape of an object, you change the drag coefficient, even if the frontal area remains the same- and that's the entire point of streamiining a vehicle.

In actual practice, people hanging square bags on the handlebars aren't the same ones trying to optimize aerodynamics of a bicycle, so nobody measures drag on stuff like this. If there was large difference, you might be able to discern the difference with a power meter. If the difference was rather modest, probably not.