Chris,
Some basic physics: The noise you see in images is mainly photon noise due to 4 things:
1) the amount of light gathered by the lens (the diameter of the aperture), 2) the light is spread out by the focal length, 3) the pixel size, and 4) the efficiency of each pixel.
As the number of pixels (detail) on a subject (linear dimenstion) is proportional to the pixel size, if you change cameras with a different pixel size, then the detail changes too. So going from a 7D with its (small) 4.3 micron pixels to a 1D or other camera with larger pixels to improve signal-to-noise ratio, means also reducing detail. If you then add a TC to get back that detail, then you have less light and you move back to the same signal-to-noise ratio (modified by pixel efficiency).
What all this means is that for a given detail on a subject, the signal-to-noise ratio comes down to the aperture and the pixel efficiency. The 7D has the highest pixel efficiency I've seen measured (in the Canon line), with the 1DIV almost the same. Going to older cameras, loses efficiency. See Figure 10 at:
http://www.clarkvision.com/articles/...mance.summary/
The advantage of moving to a 1D series camera is not in the signal-to-noise, but other performance, including AF at f/8 and faster AF, as well as many other things, like frames per second, 45-point AF, weather sealing, etc.
If you want the real gory detail, check out Etendue (also called the A Omega product):
http://en.wikipedia.org/wiki/Etendue
Roger