But concerning clean shadows: Your wish is granted instantly, because the 1dx/6d sensors are a *big* leap from the 5d2. I recently shot with a 5d2 from a friend of mine and was stunned how much banding it produced after postprocessing operations that I do with the 6d raw files all the time.
The 5D3 was released after the 1DX so just because a camera is a "later release" from Canon means nothing. The problem was the 5D3's sensor is more like they took the 5D2 sensor and just put it back in the oven to reheat. Will Canon do that again? Wait and see.
The MkIII is not too bad, really. I find it gives me about one more stop than the MkII and much,much less banding. Mind I shoot in MRAW/SRAW1 with both bodies so this includes the downsampling effect (~10.5 megapixels effectively). Yes, ~10 megapixels really is enough for my purposes.