The fastest bike will be the one you feel most in tune with, both with components and geometry of the frame.
Things such as gearing, tires will only benefit the rider if the rest of the bike is right for you.
As for flat bars, I've moved away from them due to an elbow injury last year and would never look back even if I was able to ride them normally. There is more freedom of adjustment with drop bars and the hand positioning is safer with the neutral wrist angle. It's a must have for fast rides over a variety of different surfaces.