What I've read from people who sound like they know what they're talking about is that the 168gr SMKs in particular have a design problem where their transonic profile is especially not good. Other 308 bullets are more predictable when they go trans- and then sub-sonic - this is one of the reasons you pay all that money for the fancy version of Applied Ballistics with the transonic bullet profiles.I am by no means an expert, but this is my understanding...
The trajectory is predictable as long as the projectile is supersonic; once it is subsonic, all bets are off. EX- In the same conditions, a bullet travelling at 3000fps at the muzzle will repeatedly have virtually the same velocity at 100yds, 200yds, 500yds, etc- until it becomes subsonic. At subsonic speeds, the trajectory is much less repeatable, thus much less accurate.
Most .308 bullets leave the barrel faster than 6.5CM, but they can't maintain velocity as well.
I don't know what I'd bump up to from 308. Probably something like 300 PRC or 338 LM. I don't think 6.5CM really gets me far enough into the next range category to be worthwhile.