Sight plane on a shotgun is indeed a factor - in terms of how well it swings and follows thru.
You should never look at a barrel after you mount the gun ...but any kind of rifle sights on a shotgun are a big time detriment. Your eye acts as the rear sight on a shotgun - that's why we talk about "Fit" so much - the gun needs to "Fit" so it hits where you look. Mid rib beads - front beads - are used only to check the mount ( to make sure the gun is not canted as you mount it ) ...then ignore the beads as well.
I have yet to see anyone with rifle sights ...or a barrel under 26" long on a shotgun shoot skeet or trap consistently with scores in the mid 90's out of 100. Mid 90's are not great scores / they're average scores ...but I have yet to see anyone do it - and if and when they can in fact shoot in the mid 90's to a 98 average with these 18" barrels .. I may start hacking off the ends of my barrels ...but until then I will continue to stick with guns in the 30" and 32" barrels....in the field - or in Skeet, Trap, Sporting Clays...
But if you like shooting 18" barrels for skeet go ahead ...or with ghost ring sights or whatever ...( and if you can shoot in the mid 90's with it / I'll congratulate you just like I would anyone ).