So, when you do get round to doing your testing, make sure you compare both with a 1.4TC and crop the 1Dx, you might be surprised.
Probably not. The reason I haven't been terribly motivated to set up the test is that I've done the test between the 7D and cropped 5DII (about 18 months ago, now), so I know the only difference is MP not IQ, and the 1D X is better than the 5DII for sensor IQ. I assume the test with the 600 + 1.4x on both bodies would show the same, or an advantage to the 1D X at higher ISO. That test is less relevant now that the 1D X supports f/8 AF - not much difference in pixel-level magnification comparing 2x on FF to 1.4x on 1.6x crop. I disagree with natureshots that the 7D + 1.4x will optically outperform the 1D X + 2x. That might be true with a lesser lens, but the MkII supertele lenses just don't take that big an IQ hit from a TC, even a 2x (and keep in mind that the 600 II + 1.4xIII beats the 800/5.6 for IQ). As for AF, while the 7D's 19-points are very good, the center point of the 1D X is better. But the real kicker is that most times I've been out shooting with the 600 II, my ISO has ranged from 1600 to 6400. The bottom of that range is ok on the 7D, but the top end just doesn't cut it on the 7D.
For those reasons, I'm pretty sure the 7D gives me no advantage over the 1D X, other than a few more MP (and not really all that many more, comparing the 1.4x on the 7D to the 2X on the 1D X.
The question I suppose I'm really asking myself is, do I want to keep the 7D as a backup body? Or should I take what I can get for it, now, and put that money toward a 24-70 II?
So, between 20 and 30 yards, the 7D will work and the FF won't resolve. ... So your test shots depend on what you are photographing and how far away it is.
Would you expect that to be true at, say, ISO 6400?