That is not necessarily true. In some cases, the compositor maybe configured to sample the input buffer directly from YUV420, applying the transformation on scanout and thereby saving memory bandwidth to read from the framebuffer. This makes a tremendous amount of sense when the source is a video, but much less sense when the source may be rendered vector graphics, which generally look pretty bad with subsampled chroma.
Sure, if you can use a YUV framebuffer then that can save memory bandwidth during scanout (though the conversion to RGB still happens because the screen is not YUV). But that doesn't apply in the tiled page background case I mentioned, as web page contents are composited in RGB.
