I'm not sure it's quite fair to extrapolate the performance of current lenses designed for digital sensors with very recent designs which tend to be more retrofocal layouts.
IME the 28mm f2.8 AI-S, one of the best SLR 28mms, was not worth the hassle of giving up the positives of the plastic fantastic 28mm SE Z lens. The 28 summicron v2 (11672) on the other hand, isn't much bigger on a Z camera than an M, and is just stunning. I appreciate it is expensive but again the voigtlander 28mm nokton is supposed to be as good or maybe even slightly better (if you get a really good copy) as that and is not a huge amount of money. I would probably not buy a 28mm summicron unless you also want to buy an M10R or M11, but a second hand VM 28mm f2.0 version 2 at 5-600 dollars for someone who really really wants to do manual focus on the Zf seems reasonable.
Add in the cost of a good adapter on a cheap 28mm, and like, you're close enough anyway? Plus then you have a lens that used to be small that's now as big as some of the better native Z lenses. I dunno the Zf is a 2K USD full frame camera body so if you're not upgrading vs the 28SE I don't personally see much point. But this is just IMO.