You could use an AssetBundle to download assets exactly as you imported them into Unity. Why do you require your textures to be in 16 bit format?
Edit: AssetBundles are indeed a Pro only feature. But, to save memory, compressed textures are much more useful then 16-bit textures anyways, as they are smaller and natively supported by the GPU. compressed textures can be loaded using WWW.LoadImageIntoTexture!