Power transfer won't be lost through flex (Or at most the smallest most imperceptible loss). Power transfer loss is caused by drag (bearings,chain, brake rub, tires, etc.) and usual power loss to overcome a mechanical enegy transfer system (i.e. drivetrain). The only thing flex is going to cause is a possible shift issue (ghost shift usually) , a delay in the transfer of power to the rear wheel and, for some, a strange feeling when pedaling hard.