Quantifying how biological systems maintain organization far from thermodynamic equilibrium remains a fundamental challenge. Biochemical energy converters operate under stochastic fluctuations while maintaining tight mechanochemical coupling. The stochastic-dissipative variational structure developed in Part I predicts that such systems evolve toward a minimum average action per productive event. The precision of mechanochemical coupling is reflected directly in the near-invariance of the event-level action budget. In the limit of many repeated events, empirical cycle statistics converge closely to the theoretical ensemble expectations. For ATP synthase, the action associated with an elementary chemomechanical step can be obtained directly from aggregate energetic and kinetic observables, without reconstructing microscopic trajectories. Across a broad range of conditions, the product of energy transduced per step and its duration remains nearly invariant, revealing the iso-action reciprocity expected for tightly coupled chemomechanical machines. Using Nath's macro-level OXPHOS measurements, we find that the Average Action Efficiency (AAE) per elementary chemomechanical step is of order unity, corresponding to an average event action close to Planck's constant. This scale reflects the quantized, discrete chemical and rotational transitions that constrain the enzyme's operation and is naturally interpreted as a system-specific lower bound. This alignment with theory shows that AAE is experimentally accessible and that ATP synthase operates with the predicted invariance and near-maximal efficiency imposed by its architecture. Such optimization may represent a physical organizing tendency that complements random variation and natural selection by shaping the conditions under which evolutionary adaptation unfolds.

