I would think event distance would have an effect on it. If, god forbid, I ever did an Ironman, and probably even a half inronman, I don't think I'd use the TT helmet. 1) because of the heat over time, and 2) my bike speed would be lower so that there would be less advantage, at least on a mph basis. 25-26mph in a srpint distance is getting into territory where aerodynamics are really significant. 20 mph over a longer distance, less so.