A peep is a great sight on any gun! The issue with a SxS rifle is how to get the rear aperture close enough to your eye. The slickest set up I've seen is on one of Clair Kofoed's drillings.
He had a little flip up aperture put in in place of the top lever screw head. I have a picture some where and will try to dig up.
As for the German guns with the flip up apertures...they are NOT peep sights they are designed to let you see the actual fixed sights better by increasing the depth of field that your eye can see. Keep in mind the elaborate Euro guns were designed for old eyes! (The old guys with all the cash to afford those funky guns!)
