1D X vs. 7D? No contest. ISO 12800 on the 7D = a speckled, noisy mess. ISO 12800 on the 1D X = a usable image. Comes in very handy with an f/5.6 or f/8 lens or TC combo.
Well if we apply some good old DxO type summary evaluation (meaning it will have little to do with reality)
The 1D X processors are 4 times faster than the one in the 7D, and the 1D X has 2 of them. So that would be 8 x better.
But then you take the faster frame rate the 1D X is 1.5 x better than the 7D.
Then take the improved ISO, definitly 3 stops so 3 x better.
And finally take back the the 2.58 pixel density of the 7D.
So 8x1.5x3/2.58=13.95 The 1D X is 13.95 times better than the 7D.
Does it make sense? As much as if you think you would see a full 2.21 times increase in resolution using the 7D vs the 5D II.
LOL. Good one.
Humor aside, resolution is resolution. It is a rather simple spatial construct. Assuming you did not take care to address the *needs* of the 7D, sure, it is highly unlikely you'll realize the full 2.21x resolution benefit the 7D has to offer. However, that does not change the fact that the 7D DOES offer that benefit, and when you use a good lens, with solid 4-stop IS, and/or a stable tripod, the chances of realizing a close approach to that 2.21x resolution benefit are very good. If we take the moon as an example, I always set up my tripod as low as it will go, with legs out wide for maximum stability, on windless nights whenever possible, and I use a wireless shutter release with mirror lockup to actually take the photo. Assuming I photograph the moon high in the sky on dry nights when it is center of the lens, I believe I can easily realize around 2x of that 2.21x reach benefit.
If you don't think you can, or don't care to, properly utilize the tools in hand, you should probably be using different tools. It is definitely easier to get "sharp" photos with a FF sensor that has larger pixels. That just follows the line of reasoning regarding pixel size, the diffraction limit of the sensor, and the effects of camera shake.