As mentioned earlier in the thread, the rendering problem seems related to the number of pixel draws rather than just the number of object draw calls - thousands of small objects might demand less of the gpu than hundreds of large objects, etc, so I would be interested in a bunnies test that uses a 512 image side...
I was thinking that this could be an artefact of the back to front renderer - thousands of pixels being drawn only to be subsequently drawn over many times per frame. I don't know if there are smarter ways to draw layers of objects in 2d using techniques like culling or even pasting areas that are static onto a canvas object to negate some of the re-draw load. I can't think of an easy way to implement such a thing, it might take a lot of work for negligible savings on multiple parallax layers and it won't work for animations. Just thinking aloud....