Hard evidence, not really. Drop Bars have been the only choice for Road Racing for many decades. Slight variations of Drops ep. Randonneur, touring, dirt drop ... are also popular for long, more comfortable distance. They're shaped slightly spread-out.

I've the opposite experience than you. On one bike (more actually), I went to a sort(s) of "upright" and found that I'd lost some hand positions and feel more secure and used to drops. I'm not nudging you to stay with Drops or anything. You may want to give them a chance. My friend's done centurys on his Mountain Bike, flat bars.
NO question that drops provide a much more aero profile, it's not just your impression.