East Wind, I can gaurantee I could produce a grain free, sharp 20x30 with either the 1DX or a 5D3, at ISO 1250, 1600, 2000, 2500, 3200, and possibly 4000. And if the image is of an animal where the animal in the middle of the picture, is the only thing in focus...then I could go as high as ISO 10,000 on the 5D3, and 12,800 on the 1DX. I could use only CS5 to do it, although ideally the higher ISO shots above 4000 would need NR better than PS (for those people that need to look at the print from 8 inches in front of it).
Neither camera's native resolution would be printable at 300 dpi, for a 20x30. That resolution, is 54 megapixels. I know because I recently scaled an image of mine, and produced a 20x30 print for one of my customers, shot with my puny little 15.1 MP 50D, at ISO 320....with a 200mm f/4 lens, at f/4.5. (full frame equivalent 280mm). It is hanging in their lobby, for all their customers to see. It is sharp almost to the corners, and doesn't look like it was scaled up. I used the standard bicubic, amongst other tweaks.
So again, you need not limit yourself to ISO 1000. And again, the true test, would have been a comparison between the autofocus performance of the two bodies, in low light. Hindsight now, of course.