Are you sure about the RX 480 figure? GCN SI is 1/2 rate for non-blended writes, or were you referring specifically to blended 128bpp writes (where 1/4 rate is correct).
Indeed, I believe it's the tiling modes available that's the sticking point, but I can't really go into any more detail.
DX11.2 won't allow 3D Tiled Resources at all. You could attempt to emulate one by creating a 2D TextureArray Tiled Resource, effectively using the 'slices' dimension as a 'Z'...
Volume Textures, 3D Textures and Texture3D are synonyms. All GPUs for at least as long as I've been doing computer graphics have supported 3D Textures.
What Volume *Tiled* Resources introduces is the ability for 3D Textures to be used as Tiled Resources (aka Sparse Textures, aka Partially...