Afraid I really don't know, this is going to take some trial and error. I take it that your video card drivers (on Windows) are up to date with the appropriate ones for Bootcamp?
I think I would be back to Windowed Borderless though and trying different combinations of computer vs. game resolutions unless you have done that and by now run through every possible combination that makes sense and doesn't make the game look ridiculous or stretched out.
Edit: @puzzlezaddict also plays by way of Bootcamp and is on a Steam install, maybe she has some suggestions to share.
