One option that has not been discussed here is a custom one piece die. Such a die will generally be able to exceed the concentricity performance of any bushing die that I have tried, or had reliable report on. By specifying a neck ID for the greatest amount of neck tension that will be used, less neck tension can be produced using expander mandrels, which perform very well when only small amount of neck expansion is being done. Hornady, and Whidden both offer this option. A friend has a custom one piece Hornady die that is designed to produce .003 neck tension without the dies expander in place and .002 with it in place. In either case the runout, at the end of the case neck is outstanding, well under .001. As a final point, we need to keep our eyes on targets as our primary reference for reloading tool performance and understand that there are points beyond which measurable differences in our ammo do not make a difference there. IMO it is easy to become measurement obsessed to the point that exceeds any performance requirement, or observable difference on target.
That sounds good except as bullet diameters change like in the case of Berger bullet I have seen it change up to a .001 on 6mm.. so without a bushing change and or returning necks you are screwed. Run out with a bushing die is a thing of the past... jim