Thank you, Armand, Bill, and John. To summarize: the 24-70 f/4 is better than most mid-range zooms. But when it comes to sharp corners even stopped down to f/8, it depends ...
You are asking about my reference, John. I currently use the little Sigma Merrills for landscape, in particular the DP2. Nicely clean images, good even sharpness across the whole frame. (I stopped using the DP1 as the corners are somewhat weak, at least with my copy).
So if I move to the Z7 to gain a larger shooting envelope, I need lenses that will not make me miss my DP2. A mid-range zoom would be very practical - as long as it does not make me cringe at its corners. From what you say, the 24-70 f/4 version does not seem to be a reliable candidate.
But maybe the 24-70 f/2.8 is. What would you say from your experience, Bernard?