Hi, I got also a 2017 iMac.
It runs great on windows 10 in bootcamp with high fps.
desktop resolution is set to 1920x1200, and arma as window 1920x1080.
The big fps drops appear if I open de map ie. FPS drops way down. (ie. from 60+fps to 10-20) When I click outside the arma3 window, and click back inside, fps is restored.
You can do it quickly but is annoying and hope for a fix.
all settings ultra I thought. (sometimes when fps drops, I go to settings and do autodetect again.)
disable atoc and bloom manually if necessary.