Puffin, I understand your concern.
Here is how I see it to answer your question:
Either you are shooting super accurate rifle with tight custom chamber, on bench rest ,and then going with something else than top quality brass like lapua is a false economy. That lapua brass will be already very good in neck concentricity, thickness and tension. So the neck turning you will do on it will just make it perfect. And the small irregularities will deseapear after the neck turning.
Or you are shooting a hunting rifle ( custom or not) but with a standard chamber : if you buy good quality brass for it then you do not need to neck turn, or you have a more average brass and then you could see an improvement and a justification for turning that brass to make it more consistent.
I turn my brass for my bench rest rig (30 BR), used to for my 6.5 creedmoor when I was necking down from 308 brass, now good factory brass is available from Norma.And I necked turn a small batch of 300 whisper and 308 for my subsonic reloading, and see better consistency on target.
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