The quantitative ability of a kinetic-theory-based, two-fluid model is demonstrated in a clustering (unstable) gas-solid system via highly resolved simulations. Unlike previous works, this assessment is validated against ideal computational fluid dynamics-discrete element method data to minimize sources of discrepancy. Overall, good agreement in mean-slip velocities is observed with relative errors less than 20% over a mean solids concentration range of 0.02-0.25. Local concentration gradient distributions are also studied, showing a distinct shift toward higher gradients at higher mean solids concentrations which is proposed as the bottleneck in obtaining grid-independence rather than the cluster length scale. (C) 2015 American Institute of Chemical Engineers