I think that, for most of us, the difference in resulting case capacity is less than we would notice...
The wind, shooter error, neck tension (as you mentioned), barrel harmonics, all combine to make for a soup of variables to the extent that I think it comes down to trying to split hairs.
To know, you would have to be shooting under some really controlled conditions, where you have eliminated every other variable.
I recently bought some 6.5 Grendel brass for my son's AR- made by Lapua for AA.
I will admit I was amazed at the consistency of the weight of the cases. Rarely even a tenth of a grain. Were they any more "accurate" than the Hornady brass? Who knows... at 600 yards, who can tell why one POI is an inch apart from another?