The paintings will be using a different mechanism to achieve more selections. The new mechanism the paintings will be using will require more storage space, both in RAM and map file size, and the texture (or photo) lookup will be slower, which means chunks with a lot of paintings in them will refresh slower.
I will need to see how much more storage this new mechanism will need and and how much of a performance hit it will cause before I can decide to use it for other blocks.
It might even be to expensive for just paintings, I'll just have to wait and see when it's implemented. If it is too expensive, I won't be able to expand the # of paintings. I'm hoping it won't be too expensive. And if it is not too expensive for paintings, it might still be too expensive to use it for all the other blocks that can have alternative textures, or it may not be too expensive. I'll only know the answer to these questions once it's implemented and I can profile it.
If the profiling shows it's not too expensive to use for the other blocks, then the other blocks will also be able to take advantage of it and they will have at least 255 texture slots each.