"mass is mass"
That sounds suspiciously like some hot rod mechanics I new years ago, whose mantra was "you can't beat cubic inches"
I believe the preferred term is: "no replacement for displacement"
As per the revived discussion, I'll weigh in...
Given what I am seeing out of 6.5CM factory loads and especially in the factory Winchester Match 140gr load (which is less than $1.25 per round), I see no point in choosing 308 over it (especially in 168 or 175 SMK/OTM) unless you require specific projectile for terminal performance like TAP for liability issues or unless you intend for loading for a shorter barrel and not sacrificing baseline performance. Depending on the bullets compared, terminal performance between 6.5 and 308 should be marginal at distances beyond 600y and more or less turning it into a question of what you can hit better with. As far as the "unscientific" effects on steel noted above, I have come to notice that target construction often plays a larger factor on "ringing" than the size of the pill hitting it.
If you really need to consistently put the hurt on something at greater distance, put on your big boy pants and get something that has "magnum" in the name.
I know it sucks being a slave to available factory ammo as I had to live by it most of my shooting career. Reloading (or doing custom loads from an viable reloader like McCourt) opens up a whole new world.