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.
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?