Can't speak to the 1Dx (never have shot with one) but the 7D2 works nicely with the 1.4 ext as well.
First, you have the 7D2 CF over-riding the image cropping factor to some degree ... so you get that effect, then if you still need more distance, 7D2 will produce great images with an extender.
I'm not sure how the 1Dx compares with the 7D2 without the 1.4 ext ... so, the comparison here is a bit odd. Basically a FF camera with an extender, vs a CF camera already 'extended', sorta, in once sense anyway.
As a general rule, similar quality cameras without an extender will give better IQ than with an extender -- so, you're kinda asking it to do more than it is designed to do, using a "crutch" ...
I do own the FF 5DM3 as well as the 7D2, and if I were to require the distance (wildlife, sports), I would shoot with the 7D2 bare over the 5DM3 with extender as a general rule. Sometimes, it makes sense to shoot with the extender on the 5DM3 maybe, but I've not run across it much. Most all my work is outdoors.
I'm not against extenders ... just trying to answer what you asked, not define use at specific projects, which might change the choices of lens, camera, ext or not. 7D2 is one awesome wildlife camera, for certain, and paired with the 100-400 v2, is a match hard to beat.