R2C stands for return to center. The idea is that the shifter is more aero if it's pointed straight ahead all the time. On other bar-end shifters when a shift is made the position of the shift lever changes. In either the highest or lowest gear the lever may be pointing straight down or up, making it less aerodynamic. I have fairly large hands, so compared to the size of the levers I think my hands wash out any aerodynamic advantage. However, I think best aspect about the R2C shifter is that the shift lever is always in the same position so it's more comfortable. You can also adjust the return or set position of the lever to your most comfortable position. In a long race, small things can become annoying.
Suggestion: Use a steel cage front derailleur with the Zipp or Sram R2C shifters. A steel cage (Sram Rival or Force) front derailleur won't flex as much as a lighter titanium cage derailleur. This will give you the best front chain ring shifting.