The optical gain performance of a highly Er-doped waveguide amplifier is affected by concentration quenching and cooperative upconversion effects. Based on materials parameters, which were previously determined for Er-doped sodalime-silicate glass, we calculate how the gain performance can be improved by reducing these limiting processes. For amplifiers with an Er concentration exceeding 0.1 at.%, the Er de-excitation rate is dominated by cooperative upconversion rather than radiative decay or nonradiative decay due to coupling to defects. The effect of lowering the waveguide loss and reducing the optical mode diameter is also demonstrated.