I guess you have seen it already. - The cat is out of the sack.- The D600 is definetly on the same level with the D800 and thus outperforms the Canon 5D Mark III.
Best example ist the DxO Mark value for the 85 mm primes. - The D800 and D600 reach values of 40 with the best lenses. - The Canon 5d Mark III's best value is at 35. 5 Points are quite a noticable difference in IQ, I suspekt.
And at that one should also take the price tag into account...
I'm extremely happy with my D600.