I did a series of tests with a 7D and a 1Ds MkIII (with slightly less resolution than a 5D MkIII), I did "real world" tests and "bench" tests, that I have shown before, so sorry to the regulars.
Even when the test situation was set up to favour the crop sensor in a focal length limited situation with base iso, heavy tripod, cable release, 10X Live View manual focus, flash illumination, optimum aperture etc etc I still found there to be very little difference between the two same size crops. Bare in mind these ideal test situations are so divorced from the reality of actual shooting situations (unless you only do still lifes) it is comical, in the "real world tests there is zero resolution difference in focal length limited situations. Well those were my findings after actually testing it all myself to my own satisfaction, which was a shame because I wanted to buy a 7D but realised there was no advantage to me doing so.
Here are the bench tests, first image to the left, complete ff image with crop camera image overlaid in red box, images on the right are both cameras cropped, 7D to 100% and 1Ds MkIII upscaled to match the pixel numbers. You can argue the resize but that is how I would use the cameras so that is what I did.
Second image, same setup but a human hair that i happened to notice, the 7D is around 200% enlargement and the 1Ds MkIII is well over 300%.
Now some have said there is a big difference in these two, I can't see it, the 7D does have a slight resolution advantage, but nothing much, and these are ideal bench test situations set up to show the biggest difference. Introduce AF into the equation, shoot higher than base iso, less than ideal light, handhold etc etc, and what small differences there are disappear.