Any noticeable improvements ?
I look at it kind of like two seperate systems:
One would likely pair the 16-35 F4 IS with a full frame sensor because that's how you would be able to utilize it's wide angle.
I sold my 5dIV / 16-35L combo as I only was doing landscapes maybe 10% of the time, and wildlife 90% of the time.
So to me, it simply depends on what you're after. I like the 80d + 100-400 II for wildlife / all day use, but, unless I was going to buy another full frame setup, then I'd def. get that 16-35 F4L.
We'll see! I have other battles I'm facing @ the moment, like a 7+ year old NEC PA271w monitor that's ready to be replaced, so a 38" LG is looking like the next thing in line as this one is starting to dim/color shift, on the other hand, if we have this supposed recession in the next 24 months, you may find some UH Mazin deals on stuff people don't need.