|GaN has a slightly better bandgap than SiC. Which makes them very similar in efficiency. Below is a slide from Cree's Palmour showing the difference between Si and SiC. Notice that the delta efficiency is similar to what you posted.|
GaN goes on a bunch of substrates and it has always seemed to me that the transition between substrate is determined by the substrates thermal conductivity. The higher the thermal conductivity then the higher the allowable power density. This gets discussed more between GaN-on-SiC and GaN-on-diamond which are at the very high end. I am not exactly sure what the power density limit is for GaN-on-Si or if some other limit gets hit first.