I have read about “single sighting planes” just about any time I have read through discussions about O/Us vs SXSs. How valid is the Sighting plane rationale?
It is entirely valid, for every firearm, however it is also often misunderstood.
It is simply the line your eye looks down when sighting. Shooting with sights, the line goes from your eye through both sights to the target. With a shotgun its the same, principle, but a more coarse application. You look along the top of the gun (down the barrel with single barrel guns or O/U) over the bead to the target.
One does not focus on the sight (bead) the way you focus on open sights, you look along the sighting plane with focus on the target and the bead is your "muzzle reference indicator"
Arguments about the superiority of the Single Sighting Plane come from people who, for reasons I cannot fathom, think that when using a SxS there are two sighting planes because there are two barrels. I see only a single sighting plane when shooting SxS guns, and its between the barrels where the bead is. (often on a rib).
I've never heard anyone argue over the sighting plane of a double rifle, which are nearly all SxS. I see no significant difference between them and SxS shotguns as far as the basics of aiming are concerned.