Yup. I am totally convinced that you don't ONLY get a better body and more custom functions for the huge extra amount of $ you have to fork out for a 1D body. Image sensor, RGB and AA filters, parameters and processing should be better and more controlled, resulting in an overall better rawfile on many levels. Also, I find the 1D images (1Ds3 and 1DX) to be a bit "clearer" and crispier from scratch than anything from the 5D3 or 5D2. Fine details are a tad better rendered. The difference isn't huge but it is there.
As for the underexposed part; I have an example where I tested the shadows of the 1DX vs 5D3. I underexposed the two equally and then lifted the shadows. The difference in IQ is quite remarkable. Almost like one of those Nikon vs Canon shadow noise examples out there. I'll see if I can find the test images. So yeah...the 1DX sure can take more beating in post because the files are better built. For sure...
Just a bit of a tangent...
I recently picked up a minty and complete original 1D for a very reasonable price. Different sensor tech that still has a following and the price was good. Enough of a reason to see what the fuss is about.
The images are way better than what I would have thought from a camera with only 4 megapixels- I was thinking 4MP point and shoot image quality. I really don't see an issue printing up to 13" x 19". Maybe more, I am still messing with it.
This camera outputs TIFF and JPEG. I had to reformat a card so it works- it wouldn't recognize a CF card formatted in a CR2 camera.
Canon's DPP works just fine with these TIFF files. Just a little bit of sharpening and it looks great.
So, why did Canon switch RAW formats when this camera from 2001 puts out a nice TIFF file? (end of small tangent)
Well, apparently, the TIFF format sucks when things change, like camera settings. See the short rant here in the 4th paragraph: http://www.sno.phy.queensu.ca/~phil/exiftool/canon_raw.html
So, I guess that someone at Canon early on recognized the limitations of the TIFF format.
This previously linked page (http://lclevy.free.fr/cr2/) has a whole bunch of stuff that is over my head, but I can pick out parts where portions of the code are used to identify things like the camera model.
There's also a bunch of blocks with a "?" in them.
Given that Canon can easily cripple camera features with firmware, it is easy to assume that there is a bunch of stuff happening between the tags in a CR2 file and software like DPP and therefore why some feel that DPP renders the Canon image better than other software.