I have nice low noise shots taken with the T2i/550D at ISO 6400. Usually I try to avoid going higher than 6400 because noise does creep in at 12,800. Last weekend I was shooting with a 5D Mk III, in a church without flash, at ISO 1000. Image quality was sufficient and I had some shutter speed. With the T2i, I would have been unhappy because I would have been at 12,800 with noise or at 6400 with less shutter speed. I was shooting at f/2.8 to isolate individuals. If I had wanted a group shot, f/8 would have been a better choice but in that light, shutter speed would suffer.
Selfie time! I don't know if this will help or just add to the confusion. Three photos, all shot with the 70-200 f/4 L non-IS lens. All at f/8, all with the same strobes, at the same power and placement. The lens was at 70 mm for all three shots
This is with a 5D Mk III, I left some headroom because the next shot is going to be with a T2i, without moving or adjusting the tripod
This is with the T2i from the same place
Sort of worked. Hair got cropped.
So, I moved the tripod back a little over 5 feet and aimed again.
Not super scientific, but I'm framed about the same as in the first photo.
Part 2, don't know if this will work, here are some 100% crops all the same area, one from each of the above photos.
5D Mk III on left, and ...................................................................................................T2i on right
T2i from further back
So, it looks like the 100% crops worked as expected. The T2i appears to be magnified, but not by 1.6 because the T2i is 18 mpx and the 5D Mk III is 22.5 mpx. If I used my 30D instead of the T2i, the crops would be about the same size because the 5D Mk III has almost 3 times the pixels.
In the crop from the 5D focus looks soft. It's hard to get focus exact when you are in front of the camera!