I borrowed a friends 5d3 for a few days because he's been bothered by the IQ. I've shot lots and compared images taken with it and my 5d2 and the 5d3 look softer and gives some images a plasticky look.
The owner still has his 5d2 and his thoughts are the same.
Every shot taken by the 5d3 needs to have to be put through high pass filter in photoshop to make them look as sharp as the 5d2 taken SOOC.
Post your sample shots and your setup details