Just some think-tanks ...
5D3 is most promising for raw video... sure... but what's the point to squeeze out raw 1080p video (with so much efforts and sacrifices) from a camera that costs the same like a blackmagic, where blackmagic has a hassle-free (no single images to process one-by-one) 4K raw ? How many people will buy a 5D3 instead of a bcc for exclusive video purposes ?
5D3 is most promising for raw video... sure... but what's the point into pushing all the forces exclusively on this direction, when you can clearly see the results of the poll "what camera you have?" here in this forum, but neverthless it is continuously said "550d is too old...", "550d is not worth the efforts..." ... and so on ?
Sure, 5D3 is the main benchwork to concentrate on, in order then to port to the rest of the models. That's the right way.
Sure, 5D3 has a larger sensor so more light sensitivity than a bcc (not so much more, however). But as an advice: don't understimate the impact of even a SD resolution raw video achievement on the "poor" 550d !