If resistance is indeed lower and heat is dissipated more efficiently with same current capacity, it means the efficiency is higher. It sounds a great improvement to me. Any further information and insight?
I am not aware of any SiC or GaN 300mm wafer supply... In fact, I don't even think you can get 200mm substrates with those materials. I think you can grow SiC epi on a silicon substrate, but I doubt that is an efficient way to make power devices. I'm no expert in power, though.