Bis, I have the same trouble with my 30-30 when I neck size only. I use the Lee collet die for neck sizing. If you are using a full length sizing die and only partially sizing, you may actually be moving the shoulder forward slightly and this could be part of the trouble. What happens when we "neck size" in a full length sizing die is not only to size the neck, but a portion of the lower case body is sized as well. In resizing, as the brass is shrunk to unfired diamensions, the brass must flow somewhere, since it can't go out, it can't go down, it must go up, or get longer, therein is the increase in length. Your 243 headspaces on the shoulder, and since it is longer now because you have not truely full length resized, it more completely fills the chamber, good for accuracy, but bad for a weak extraction system.
On firing the brass is streaching to fill the chamber space, and doesn't relax as much as true full length sized brass does, causing stuck cases. If your experience is like mine, the more you only neck size those cases, the more of them that will stick in the chamber.
You are correct, you may see a difference in accuracy or point of impact between your neck sized cases, full length sized and factory new cases. You also may not. I would suggest that you run a few controlled experiments to determine this.
Neck sizing is usually done to either increase accuracy potential or to increase case life. it may or may not increase accuracy potential, I have yet to be able to prove this in my 30-30. It has increased case life, but as cheap as 30-30 brass is, or 243 for that matter, it may well be that the best solution is to full length size if this eliminates a stuck case problem. I like the Lee Collet die for neck sizing because the cases do not need to be lubed. This makes my reloading simplier and quicker, since I have a backyard range, I usually load up five, shoot 'em, load and shoot 'em again for a few hours.
Well, I've rambled enough, hope some of this helps out. Sorry for being so long winded, JP