What's a Stop Worth? (Lens Advice and Discussion)

Disclaimer: I apologize if this is not the correct way to post a topic I wanted to discuss. I think the correct way to do that around these parts is a "new article" but I am not 100% sure.

Disclaimer #2: Yes, I know this is a topic that has been debated to death, and the last thing that anyone wants is another "what lens should I buy" thread, but since things are new here, it seemed like the time to bring up a perennial "favorite" topic...

Disclaimer #3: This is long and rambling, but if you want to get to the question, just skip down to "Get to the Point"

I have been a Nikon shooter since the early 2000s. (Grew up on film, but shot a lot of different cameras until the digital era, but that is a post for another time.) My Nikon body journey went: D70 -> D200 -> D700 -> D750 -> Z6II. In that time, I usually owned, as my first lens with any given camera, so sort of variable aperture super zoom. The most recent of these was the f-mount 28-300 f/3.5-5.6 VR. They were... fine? Great for walking around on vacation, particularly outdoors, but not great in low light or indoors. But as the bodies got better, and the high ISO didn't look too awful, they were ok.

When I got my Z6II, I once again immediately went for a superzoom; the 24-200 f/4-6.3 VR. It is, again, fine? Optically speaking, I think it vastly outperforms any of my previous f-mount superzooms, but I still felt it was missing a certain something.

So I got the 24-120 f/4. This is a superb lens! Handles great, images are quite sharp, focus is nice and snappy, and its minimum focus distance makes it the next best thing to a macro without having to carry a dedicated macro. Seriously, if you have a z-mount camera, you should probably own this lens. But I am getting sidetracked...

I do sometimes shoot indoors with terrible lighting at things like concerts. Around the time I picked up my Z6II, I found a used f-mount 24-70 f/2.8 E VR G-type lens. (This is one of the f-mount "holy trinity" of 2.8 glass.) This lens is also great, but dear lord is it heavy and quite unbalanced on the ftz adapter.

Just comparing the 24-70 f-mount to the 24-120 z-mount, I think these two are pretty close in real-word (not shooting test charts in a studio) in optical performance. But the 2.8 is 1 full stop faster. But I often don't want to shoot it because it is so much heavier than my z-mount glass.

"But wait!", I hear you cry. Why not just get the z-mount of the 24-70? Well, because that is a bit out of my budget at present. And I would also like some extra reach for concerts, to be honest. I think the solution to my personal dilemma is the newly announced 70-180 f/2.8 z. I think it will check all of the boxes for what is currently missing in my setup.

But I did consider the 28-75 z-mount. But this lens is, by all the tests and reviews I have seen, slightly optically inferior to the 24-120 z. I gains me a full stop, but at the expense of softer corners.

And that was when I started to really think about the question I would like to hear other folk's thoughts on:

Get to the Point:

So, what's a stop worth in this age of modern IBIS (In-Body Image Stabilization) and reasonable looking high ISO images? Is a f/2.8 vs an f/4 really all that big of a difference in this day and age? It is only one stop, and a bump in shutter speed (with IBIS taking up the slack) or a bump in ISO (with sensor performance lending a helping hand) could make up for that.

Sure, the f/2.8 gives you slightly softer background than the f/4, but particularly if you are at the longer focal lengths, does it matter all that much?

Is one stop worth $1,000 and a bunch of added extra weight?