I've been thinking about some of the same issues the OP has. Perhaps a different way to ask the question is this.
At what barrel length does one hit the point of diminishing returns? For example - and these numbers are all pulled from thin air - if there is a 100 fps increase from 18" to 19" but only a 50 fps increase from 19" to 20" I would say you've hit the point of diminishing returns.
At what barrel length does one hit the point of diminishing returns? For example - and these numbers are all pulled from thin air - if there is a 100 fps increase from 18" to 19" but only a 50 fps increase from 19" to 20" I would say you've hit the point of diminishing returns.