I have a YHM 7.62 QD Titanium Phantom. Use it on my AR10 clone and all my AR15's. It actually sounds TO ME quieter on the AR15's. The exit hole is .380 in dia. which is .072 larger than the 308 dia bullet.
I ONLY shoot full power ammo through it, so the supersonic crack of the bullet will more than drown out any difference in performance. For those who have never experienced suppressed fire in person, let me tell you the bullet makes a LOT of noise when it breaks the speed of sound. Sounds just like someone firing an un-suppressed .22LR with high velocity ammo. I recently stood 50 yds downrange about 15 yds away from the path of the bullet (behind a berm) and had a friend fire my AR10 from the bench. I heard absolutely NO SOUND from the direction of the bench. The crack of the passing bullet sounded like someone standing 15 yds away firing the aforementioned .22LR. The sound followed the bullet downrange. I'm sure if the bullet wasn't making all that noise, I could have heard something from the direction of the shooter, but all the noise I heard came from the point in the bullet's flight that was nearest to me at the 50 yd line. Unless you were very close to the shooter, you would not be able to tell where the shot came from.
I tried some subsonic loads for the .308 just for sh%$s and grins and it was very quiet except the bolt partially opened and then slammed shut because I couldn't turn the gas off. Since I have no practical use for subsonic rds I won't be using them anyway, but shooting subsonics is the ONLY way you might ever notice a difference in performance when shooting smaller calibers through a larger can, and I'm not convinced of that. Of course most pistol rds that are suppressed are subsonic anyway.