As the Title says, I am considering replacing my 16-35 f/4 with Voigländer 15mm III Native E mount (soon to ship) ; Zeiss Batis 25mm f/2 and Sony/Zeiss 35 f/2.8
What focal lengths do you use on the zoom? If you use the entire range you may be better served to keep it. If on the other hand you find yourself favoring one particular focal length then you might find that a fixed focal length works better for you. When I shot Pentax, I had a Sigma 10-20 zoom that I shot almost exclusively @ 10mm.
The plan you suggest is similar to mine for focal lengths shorter than 50mm. I plan on primes. I currently have the Batis 25mm as my widest angle lens and am keeping an eye on the new trio of wide angles from Voigtlander (awaiting reviews and comments from users). In the past, wide angle has not been a major part of my photography, but the Batis has stimulated my interest. My reason for using primes in these focal lengths is based upon the 24 MP sensor of the A7ii. It gives me enough pixels to crop images as I like in post processing instead of zooming. Of course only the Batis has auto focus in this group of lenses. BTW, instead of the 35 FE 2.8, I opted for the Voigtlander Heliar 40 2.8. I love the way classic Heliars draw. And the modern aspheric middle element in this lens brings the classic design up to date. I reviewed the 40mm Heliar here in the Adapted Lenses Forum. Bottom line is I agree with your plan and hope to see your images resulting from it someday. However I have nothing disparaging to say about the 16-35 f4 zoom or zooms in general.
This is key. Before assuming you shoot over the full range, pull up the stats in Lightroom. I did and found more than 90% of my shots with the wide angle zoom were at 16mm. I replaced that heavy expensive zoom with the 15mm voigtlander iii and have never regretted it.
Did that and most (60%) is actually at 16mm 15% 24ish or 26% 35mm nothing really in between. From my last trip selecting the 16-35 f/4 = 2000 photos, here is how I broke it down {}
How does the CV fare with starscapes wide open? It's fractionally slower than the FE 16-35, but oh so much slower. Rokinon 14 probably a better bet but not light...
