5D3's IQ is same as 5D2's. Good photographers can take any equipment and make them look good. But 5D3 itself has no IQ improvement over its 4 year old predecessor. Fact.
But...if true (big if), is that because they can do no better, or by choice?
The 1D X is very noticeably better than the 5DII in terms of IQ. That suggests that Canon could have made improvements to the 5-series IQ, but chose to improve pretty much everything else, instead (again, IF the 5DIII has no better IQ).
Since I qualify to say it, and I wasn't going to go there but you MADE me , my RAW images out of the 5D Mark III seem to have better IQ than my 5D Mark II RAW's. The shadows are much better. JPEGS even better. IQ of JPEGS is actually noticeable to a non-photographer. Not at all saying 5D Mark II had bad IQ, we all know it is excellent. This is a subjective comparison.