It's a very good question, it seems (from what I've been reading on the boards) that low ISO IQ on the Canon side has remained steady or even gone down slightly in the latest generation or two. The higher MP (overall) has generally compensated just fine as many people don't need prints out to 2'x3' or anything like that. The real push the past few years has be high ISO performance (>=1600), however from what I've seen from the DxO stuff, Canons generally nose dive on DR at high ISO, although at the extremes (6400? 25600?) I think the 5d3 has better DR than D800 and ends up having a bit more IQ.
Can't speak to the 1DX, but I wouldn't be surprised if it was at best about the same (pixel peeping) IQ at low ISOs. At high ISOs, especially extremes, I'd say it'll probably end up having overall better IQ than the last generation.
In terms of real-world impact of IQ, I'd say a lot depends on your workflow, what you do, and what you deem to be bad/better/good/best IQ.