I'd certainly upgrade my 7D to a 7D Mark II with the same frame rate as the 1D4 and much better IQ than the current 7D. I like being able to keep the 1.6x reach that I would have sacrificed to 1.3x with a 1D4 or give up with a 1Dx. If the 7D2 is half the price of a 1Dx or less, I'll buy one in a heartbeat.
Well, at ISO 800, you actually have more "reach" with the 1D4 than the 7D, because IQ is so much superior on the 1D4, you can crop beyond the 7D cropping.
It's not so easy.... you have a camera and a lens... the lens used will affect how much cropping you can get away with and the lighting as well.
A full frame camera will visibly outperform an APS-C camera under poor lighting, but under good lighting the difference is much less apparent.
If you are using a cheap lens, you will not have the resolving power of a quality lens. A while ago I compared an 18-200 against the 100L. My target was the page of a book. The 18-200 at 100 could not resolve words, and at 200 you could guess what the words were. The 100L was able to resolve the words. In other words, a good 100mm lens had longer reach than a poor 200mm lens.
I then compared a 5DII against a 60D, both using the same 400 5.6 lens, and moved the book away until I had problems resolving. In daylight, (i think it was at ISO200 but not sure) the 60D out resolved the 5DII, but as the light dimmed, (i think using ISO1600) the 5DII out resolved the 60D....
There are no easy answers that cover all conditions.