The Mark III was quite a bit better in low light in comparison to the Mark II. Not a full stop better, but still noticeable.
For me, if the Mark III had cost the same as the Mark II, I would have called a decent upgrade. But it didn't. It cost $1,000 [i]more[/i] than the Mark II at launch. That is unacceptable.
Also, for future sensors, the few tests of the 1DX that are out show a sensor with excellent performance -- true, next-generation performance. Unfortunately, in grand, arrogant-as-hell Canon fashion, they only put their good sensor in their most wildly expensive camera. Can't afford it? They don't care. They're the telephone company.