Not sure I understand your question. My decision to use the f/2.8 lens was purely for the extra stop of light when necessary, not for autofocus or depth of field, however those were both welcome enhancements.This comment has made me curious as I thought RF focused as well at f/4 as it does at 2.8. So I presume that with what you were shooting you could get away with minimal Dof all the time ?
In response to your statement about f/4 focusing as well as f/2.8 it's still a stop of light that's being lost during autofocus. So, at a certain point, you will find a situation where there isn't enough light to focus each lens. That will come at 1-stop of exposure in measured light.
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