How can the chamber be an unknown? The chamber will either be cut to SAAMI or CIP specs. There will be minimal differences between chambers, and not enough to make a discernible difference in case capacity. The biggest difference will without a doubt be in the internal capacity of the case.
I don't think it's a case of 'overthinking' or 'what rocks your boat' as reloading for precision shooting is all about the accumulation of all the small details making a difference. Some people may not care about going from, say 0.6moa to 0.3moa group size, but there are those that do, and as I said, it's the small detail that is the difference.
Jonathan.