I used my 5D2 the other day after not using it for a while, and honestly it seems like the images may be a bit sharper on the Mark II. Not necessarily better, but sharper.
That is one of the reasons I sold the 5d3 to get the 1d X, IQ really wasn't improved, except for color-accuracy, from the 5d2. It was more bang in the sharpness, and I love sharpness and detail, with the 5d2. Above 1600 iso the 5d3 is clearly better, but 100-400, I just liked the 5d2 much better.
But I also had this experience as my gf still uses the 5d2 for food-photography, and I use the 1d X, there is a slight difference between those two cameras, going back to the 5d2 feels like going in slow-mo
That being said, I always found the 5d2 extremely sluggish and slow even when new. After using the 1d3 and 1d4 (along with 5d1 and 5d2) the only reason being FF, I was sooo happy to see the FF 1dx announced. The 5d3 seemed like a better bang for the buck, but after using the 5d3 for four months and then switching to the 1d x half a year it's pretty obvious it isn't.