The non-double diamond shaped triathlon bike frame may save drag corresponding around 8 watts at a speed of 20 mph according to the wind tunnel experiments – please see article.
Such a saving in drag is worth to take into consideration when choosing a triathlon bike. But some experts also state that aerodynamics is only one of several factors to care about.
It also matters whether the geometry of the bike fits the triathlete and she/he can maintain can an aerodynamic position. Furthermore, mounting of hydration systems matter too.