The 7D has the best APS-C sensor currently offered by Canon. The features that differentiate it from the lower models are build, AF, fps, etc.
If the 7D had a larger sensor - even slightly larger - the image circle of EF-S lenses would not cover it. So...bad idea.
neuroanatomist: I recently went out shooting for a week with my brother inlaw. He has a 7d and a 24-70. I have a 550d and Tamron 17-50. Over the week we exchanged cameras several times for hours at a time. I expected his 7d+24-70 to blow the 550d away. Well, I was in for a surprise. Overall, the 550d + Tammy where at least as good as the 7d. I would even dare that in some cases they where better. When shooting at night in low light on a tripod, the 7d took quite a few seconds to digest each shot - the 550d was much quicker?
Bottom line is (my impression) that placing aside all of the 7d's pro qualities as far as AF, body, etc. from a pure IQ point of view - I saw no large advantage - and even seem to feel a certain annoying softness in the images.
Strange ? Can you comment?
Has your brother-in-law micro-adjusted the AF on his 7D with the 24-70? That should have gotten the focus right on.
Have you micro-adjusted the AF on your 550D with your Tamron 17-50? Oh, I dont think the 550D can microadjust. I know the 60D does not. That is something that was dropped during the "upgrade" from the 50D to the 60D