Frames are but one component of an entire system. That system includes wheels, bars, all components and the most important part, the rider. Since the rider has needs such as hydration, we consider bottles as part of the aerodynamic equation.
The latest S2 and S3 downtubes are shaped, not only to offer higher torsional stiffness, but to take into account the aerodynamic drag of your water bottles. The result of this down tube shape is less drag for the frame as well as less drag for the entire system including your hydration.
We have designed the S5 to be the fastest road frame with or without bottles. The S5 fits two water bottles in the usual positions: one on the seat tube, one on the down tube. In the standard 2-bottle arrangement, the bottles fit together tightly with each other and the frame, reducing the gaps where turbulence can form resulting in lower aerodynamic drag.
In addition, an extra bottle boss on the down tube creates a new lower position for a single bottle. Using the low position on the down tube saves 14 grams of aero drag, a reduction of about 1.4 Watts, compared to a bottle in the usual position. The lowest drag bottle for the S5 is an Arundel Chrono on the seat tube, which added no measurable drag in our wind tunnel testing. An Arundel Chrono on the down tube added only a few grams. In essence, the S5 is optimized for more than one option.