Can't really tell from this limited sample set, but I'm not seeing much of a difference in ISO performance between the 1Dx and 5DIII.
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(The D800 would offer 50% more MP and perhaps a solid 2.5 stops better dynamic range at ISO 100 and perhaps between 1/3 stop worse to 1 stop better performance at high ISO. Based on that, the D800 most likely does have an all around better sensor, although it is not set in stone yet.*)
Take my own example - I'm making reasonable money using an ancient 40D to do events work. This week I delivered some shots to a company who I know will just use these images in newsletters and online. Megapixels are utterly irrelevant in this fairly typical situation - indeed, the client prefers smaller jpgs than the 10MP 40D produces just for ease of sharing and sending!
Where I am losing out is in low light performance - losing shots due to poor ISO quality or to camera shake from longer than ideal exposure times, means my keeper rate is pretty lame. That's why I was holding out for a new (ideally full frame) EOS before upgrading, so a mkIII that promises to improve these qualities over its highly respected predecessor is definitely what I'd like to see.
I'm kind of looking forward to the Nikon fan bois posting 100% crops from their D800 with a 50mm lens and requiring 1/200 to get a non-blurry picture when hand held and the screams associated with that.
Guess you're one of those pro photographers who are doing REALLY well and has really deep pockets. Its not about 'compromise', its about ROI ... as in any business.
No pro would switch at this point in time, period. These are brand new cameras and completely untested. In 4-7 months, that could change. Anyone switching, based on specs and demo shots, isnt a pro in my book.
Bigger files means increased storage requirements. I've already filled several 1TB hard drives with photos from my 5DII & 7D. 40+ mp would be ridiculous!