That's true - to match framing the subject distance would have to adjust. Good point.
Even so. I'm, not sure one loses much by going to 2.8, if you are increasing the focal length - as the OP discovered.
I do wonder how much it changes between a crop sensor MF back vs. a P65+ sized sensor. Likely not all that much, but it's curious. I'm still used to what I get on the big film, so all this seems small.