Quuestion on uprezing and the 1Dx
My understanding is that pixel resolution is the same in any bitmap digital file. A digital photo is a bitmap image.
Upresing, in Photoshop or whatever, will always degrade quality because it adds 'approximate' data next to existing pixel data. The usual result is to make the image appears 'soft' when printing. If the image is not upresed and simply output at a size much larger than it has data to support (e.g. at 72dpi) then it may appear 'pixelated'.
I don't see how the image coming from one camera, over another, can affect this process. Regardless of the quality of the image in other regards.