The 5D3 resolution is (coincidentally) 3 times that of 1080p (once the top and bottom are cropped). Doing something similar for 4K isn't so easy though. Something around 26MP would give 1.5x 4K resolution, which might help a fast/efficient algorithm.
EDIT: Though I think the 1Dx2 is taking a 4K res crop from the sensor, which is another option.
Yeah, exactly, it's less about getting the biggest megapixshuls possible and more about matching or cropping correctly for efficiency. The 1Dx2 is DCI 4K crop, which roughly translates to the 1.416 or something crops of Super 35 sensors of yore (wider than 1.3x 4:3, I believe).
Exactly. Canon has promised answering each need with specific camera models that will keep each group happy instead of compromising with one-size-fits-all options.