I have run into issues using the extension on the latest legacy driver (12.6 on radeon hd4850, windows7 x64). I found 2 problems so far:
1) glTexStorage2D does not work with GL_TEXTURE_CUBE_MAP, gives invalid enum,
2) Textures allocated with glTexStorage2D do not work as depth attachments via glFramebufferTexture2D, generates invalid operation.
Any chance AMD will release an updated legacy driver?