Have used the Lyman plug expanders for jacketed bullets loads, but used a smaller dia one than the neck, which didn't expand the neck for a cast bullet, but did bell the case mouth. An immediate improvement in concentrically was observed using a standard seating die. There is another manufacturer that makes these plugs, but have forgotten.
IMO using the redding carbide (or whatever it made of) plug on their dies works so well not to pull the neck off because it is free floating. The RCBS not as much. The regular pull through plugs are locked down, but can self align some if not locked down. Some have advised letting the expander free float, till it enters the neck, then lock it down.
On most regular sizing dies the neck does not get expanded till after the brass is either full length or neck sized. If going to the trouble to not pull your necks off, would again advise to square your sizing die. If you want to debate about the benefits of squaring your dies at all, have at it. You are not debating me, but scores of others in reloading/shooting business with much more experience.
https://www.facebook.com/USAMU1956/posts/10153707187509734:0
http://bulletin.accurateshooter.com...ase-concentricity-with-standard-seating-dies/
https://www.sinclairintl.com/guntech/steps-to-minimize-bullet-run-out/detail.htm?lid=16099
Squaring your dies provides the self aligning benefit of a forester press without spending the money. Squaring the dies helps to self align it vertically and horizontally, and will recommend it for any function used by a sizing or some seating dies. Yet another manual that describes this is Sierra's 5th edition page 153. IT COSTS NOTHING
Actual experience has taught me not to assume the outside of a die is in line with the inside.
A very good point about the nicks in a rim, especially from semi's. Have found the vertical force from the full length re-sizing process on semi auto brass helps mitigate that.
Other advise is to get a decent comp seating die.