I figured I'd ask around. I drug out the old TA01 ACOG last night and slapped on 20" barreled AR just for old time's sake. And of course, the short eye relief required me to alter my cheek position. So I removed the folding rear sight, put the ACOG on as far to the rear as possible, and then re-installed the rear sight to a position just in front of the ACOG.
Unless I wanted to put it even further to the front, I mounted it backwards so it folds forward. It makes the peep perceptually smaller, but not as much as the small aperature. Just not a ghost ring any longer. Eye position on the ACOG is now acceptable. So the question is, is there an actual detriment to this? I know the Canadians commonly mounted a BUIS in front of their ELCAN optics. Rear sight position seems to be a personal preference when it comes to an RDS on a pistol. Thoughts?