A quality comp seater is never a mistake, IMO, simply because it can reduce runout caused by human element, possible press or shellholder issues and align the bullet with the case. It may not reduce runout in cases with straight necks with careful loading and even neck walls so it will NOT fix any mis-alignment caused by uneven case neck walls, just slightly reduce it.
They are also more likely to use a seater stem for VLD, tipped and match bullet profiles with long noses, which if used with the wrong seater stem can create runout. Also they are more likely to reduce shaving and runout if you use cast or miss a chamfer.
If you have cases with uneven neck walls, there is no good fix and the only real fix is to neck turn the high spots. Uneven neck thickness is likely the cause of your issues (assuming all of these cases have been fired once).
I usually cull the cases; really, I just buy brass of known quality and try to avoid cases that are "seasonal run" only.
Lapua, Norma and Nosler cases in 243 or 223, a straight sizer and a decent seater die should eliminate any sorting and large variables.
Hornady and LEE seaters also work quite well on the cheap, just be deliberate when putting the case in the shellholder.
