100% agree to all. Admittedly, my longest shot (600 meters) was with irons on my M4 on a SAW range, but I think RDS are almost always faster and more accurate.Easier than you think. Especially for absolute co witness. Zero your red dot then adjust your see through BUIS to the red dot. Shut off red dot (to simulate failure) and zero the irons. If you do the job you should be on or really close to where your red dot was hitting with your backup iron sights. We called it "lollypop" the sight.
I did not like lower 1/3 co witness. When Army doctrine got caught up to allow shooters to choose either, by picking the right risers. I shot it a few times to give it a try but stayed with absolute. And mimicked a similar setup on my civilian rifle.
John