Corona stores stuff such as multiple mapping channels, support for anisotropy, support for visible/invisible edges with edge shader, support for forestpack color, etc... I dont know how many of these features does octane support.
Also Corona does not compress textures, they should take 512MB in the benchmark according to my calculation. Octane probably uses some kind of D3D/OGL lossy compression which comes practically free on GPUs and is already implemented in the system.