The 7DII seems to handle smooth gradients and constant tones WAY better than the 7D did. That was a major issue I had with the 7D.
In any case, the 5DIII definitely shows more detail, and less chroma and luminescence noise at high ISO than the 7DII does. But man, you really have to look for it. I'm questioning if its worthwhile to pay twice as much for a refurbished 5DIII and 24-105, when I can "simply" buy a new 7DII and not have to deal with the transition to FF. I tend to shoot slow moving targets more often but still, the 7DII is compelling.
Now you done it! How dare you suggest full frame might not be worth the investment!
Seriously, even though I switched from 7D I to 5DIII about a year ago, I'd have to say you really do need to think long and hard about whether or not it's worth the price of entry to full frame. I would say it depends in part on what lenses you already own because, as you correctly point out, it's not just the cost of the body, it's the cost of the lenses as well.
Clearly the gap is narrowing and while there always will be a gap, it is moving more and more toward the margins.
Now, expect to see an avalanche of posts from full framers telling you how APS-C can never compete with full frame. But, just remember, we have to justify our investment.
Consider the cost of buying all your lenses one stop faster. A 300mm f/4 versus 300mm f/2.8, a 70-200mm f/4 verses 70-200mm f/2.8, and so on. If you struggle in low light and need to gain a stop, the investment in FF can save you money. If you are stopping down, then that's not a factor.
On the other hand, if you are focal length limited, a high MP crop camera might give you a edge over cropping or adding a TC, at least with high end lenses.
Finally, with a crop camera, those wanting to get shallow depth of field will need to purchase lenses with wider apertures, so given the same lens, FF will cost less.
In fact, a 6D can be had for significantly less than a 7D MK II, so the price of entry to FF is blurred by a lot of what-if questions.