I won't be using a full frame camera for birds and wildlife in the future. The reason is this. Wildlife is all about reach. The 1DX is amazing. It's the best all round camera.It has one major Achilles heal, reach. Ill give you an example. I was out with a Nat Geo published wildlife guy. He had an x. Every and I mean every shot he had to use an extender. I didn't. He has a choice, get closer, which shooting wildlife we would do if we could or buy a bigger heavier lens. Then again I could buy a 600 and have an even bigger reach advantage. The x is better in everything but when reach is needed, no.
Sweet spot? These lens are bang on edge to center.Dont get any vignette issues with the 7D, tiny bit 1d4, more with a full frame like my 1ds3.
Extenders, never on a 7, a bit on the 4, almost always on the full frame. It's a reach thing.That said, on a pro body the slowing of autofocus isn't so bad even with canons excellent x2 mk3 extender. Shots are sharp, just not so sharp as a bare lens.
No issue buying a 500. Couldn't hand hold the older 500 a lot like the new 600, can always hold the new 500. I also don't,won't buy an x, no reach.
Image quality? Oh dear here we go. There was no differance in quality at A3 between new and old lens. What was a differance was weight, stabilisation, flare, but print for print, no differance but you will get more keepers and sharper slow speeds with the new one, and you can hand old all day so you will get more in focus pics.
As for image differences between cameras the bigger the sensor pixels you really do see a richer pic, more contrast but nothing a minute in photoshop won't fix. But the worse the light the more you see the differance.