My primary interest is wildlife photography and most of the shooting that I do is hand held because I do a lot of hiking with my camera in rough country. I recently acquired a 7D and the EF-L 100-400 to replace my T3i and a Sigma 150-500 lens. I am astounded by the improvement in sharpness that I'm getting. Photos at any ISO are far crisper with my 7D 100-400 package than with the T3i/Sigma combination taken at the same ISO.
My point is that I couldn't imagine that there would be that much improvement over what I'm getting with the 7D in a full frame body. There comes a point, and I believe that I've reached it, where my eyes are simply not good enough to tell the difference.
Since the T3i and the 7D have the same sensor, the effect you are seing is due to the change in lenses.
The 100-400mm is much sharper than the Sigma 150-500 especialy at the long end.
I think that you will see a similar change if you upgrade to FF. particularly at ISO 800 and above, where noise impacts the sharpness of the photo.
Look at Neuro's Cardinal at ISO 6400, this is what I would expect to see from a 7D at around ISO 1600
