We performed light optical diffraction experiments on a nanoparticle-polymer volume holographic grating in an angular range including far-off-Bragg replay. A comparison of three diffraction theories, on the same level of complexity, with our experimental results shows that the dynamical theory of diffraction and the first-order two-wave coupling theory using the beta-value method fit the data very well. In contrast, the prevalent two-wave coupling theory using the K-vector closure method yields a poor fit with an order of magnitude worse mean squared error. These findings must be considered for accurate determination of coupling strength and grating thickness.