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?
The problems still exist in the 13.1 version of the legacy driver.