When dealing with directly composited images, where a single image is assigned to a layer, we still:

a) Decode the image into our CC image decode cache
b) Upload the image to the GPU
c) Raster the uploaded image into a tile (basically a copy)
d) Draw the tile

(b) and (c) can be collapsed, as we're basically generating two GPU copies of the same texture.

An approach that might solve this is:

1) Hand off the uploaded texture from (b) in a texture mailbox, and have the compositor use that directly. This is probably the most "correct" solution, allowing the image to be cached as normal. The complexity here is that the cache can now have permanently vended out images.