First off, I'm not calling anyone a liar. That said, there are some things that have been said that I am having a hard time with.
Anyway the 6.5 Saum case is near perfect because it allows you to throw a 6.5 140gr at 3100 at about 53,000 PSI with H1000. Other cases have too much capacity or not enough. The Saum case was picked not because its magical , because it is the perfect fit. In a perfect world I would take the the RCM case and shorten it a tad and give it a longer neck. That would truly be perfect!!!!
1 - I'm a little skeptical that 53,000 psi of H1000 in a 6.5 SAUM is pushing a 140 @ 3100 fps. Is there any Quickload data on that?
2 - I don't buy "the perfect fit" business. If you go to a larger case, like let's ay the 6.5 WSM, You will on average get higher velocities with more powder at equal pressure. Example. I own both a 300 WSM and a 300 RUM. When I load those cartridges to equal pressures the RUM will produce at least 300 fps more velocity than the WSM. If i wanted to extend barrel life in the 300 RUM I could reduce charge and pressure to much lower than the WSM to achieve the same velocity. The charge itself would still be greater, but the pressure would be much lower. Point being, there is nothing "magical" about the "fit" of the SAUM case.
Now reducing the load and pressure will increase barrel life, and it might even do it exponentially when you get down to certain levels. I really don't know how all that plays out. But I do know that bigger case = more velocity with equal pressure.