I think you're right on that one.
It's fairly easy to get a semi auto to run 6.5CM cleanly but it's easier to get 308 to run right since the rifle was designed to run it.
My 6.5 semi hates 139gr Superformance factory loads, it crushes the brass and makes wild groups but then using the same projectiles in my handloads it runs perfectly (hovering right around 1 moa) and I have no issues other than I'm down on velocity compared to the factory offering. I've also swapped a JP captured buffer spring system with the heavy spring in it and I've also added an adjustable gas block to be able to tune it better (haven't tried factory ammo since changing out parts).
The biggest thing is that at the distances most people are most likely to be using their AR-10 class semi auto there will be no benefit to running the 6.5CM. You'll just be paying more for ammo to do the same thing as someone running a 308, you also lose the option of grabbing a case of surplus to have some cheap ammo to play with.
I would however still suggest swapping out the barrel if you want to improve accuracy, a quality barrel would benefit any rifle.