The theoretical gain, radiative and Auger recombination rates, and threshold current densities of ideal wurtzite (WZ) and zincblende (ZB) 20 Å In0.2Ga0.8N/70 Å GaN multiple quantum well laser diodes are compared. We obtain upper bounds on device performance, which are based on reliable calculations for both band structure and recombination dependent features and show (1) that the performance of present devices having the ZB and WZ structures are within 20% of each other in InGaN/GaN, and (2) that present performance of the best currently available devices is only a factor of 3–4 below the theoretical limit. Radiative recombination is far more important than Auger processes. The calculations are performed using a superlattice K⋅p formalism and density functional theory within the local-density approximation. The latter yields bulk zone-center energies, wave functions, and directly calculated momentum matrix elements. © 1998 American Institute of Physics.