This study set out to analyze the performance of modern nitride GaN-based tunnel-junction vertical-cavity surface-emitting lasers (VCSELs) at various ambient and elevated temperatures, with the aid of an advanced self-consistent optical-electrical-recombination-thermal model. Particular attention was given to the impact that both the length of the laser cavities and the mirror reflectivities have on laser operation across the temperature range. The optimal number of pairs in the top Distributed Bragg Reflector (DBR) was determined for stable operation at high temperatures and relatively high output powers. The effect on laser performance of expanding the metallization around the bottom DBR was also investigated.