I can't profess to have practical experience of the situation but I wouldn't have a problem with using them at typical shotgun ranges. Must be like hitting them on the head with a 14 pound hammer!
If you're going to use a shotgun and slug I would have thought the most important thing is getting your shotgun (if your using one without rifle type open sights or a scope) set up just right. Don't forget that your shotgun could be set up to put the centre of a shot pattern a foot high at 30 yards and maybe higher. Thats not a problem using shot, and once your used to it you probably won't notice you are shooting a bit low on everything or the pattern will be taking care of things but with a slug, you just missed piggy! Equally, trying to shoot with a gun that doesn't shoot perfectly to point of aim left and right is bad enough with a 3 foot wide shot pattern let alone a slug.
This is one of the only cases I can see for an adjustable comb so you can play about with it to your hearts content until both cast and comb height are spot on.
If you really want to see where a shotgun is shooting, go out at night with a target board and lamp. put a single spot on the board, load up and get a mate to lamp the board, don't wait, as soon as that lamp goes on, mount and shoot at that spot. It is important to get the man with the lamp to stand at least 10 yards to the side of you so you get no overspill of light onto the gun, otherwise you might actually be looking at the barrel and lining things up, that will give you a false result. If you can't see the gun, you can't line anything up and you will know if the gun is actually shooting exactly where you're looking. I haven't done this for a while as all my guns shoot right where I want them to but it does work. It is sometimes the answer if you've got a gun you just can't hit anything with.