It really boils down to this: in reality, a 7.62 round is preffered because it rips through most commonly-found cover. So what that SAPI plate won't stop it? ... it really doesn't matter when you're blasting people literally through walls.
5.56 will perform better against armor due to its high velocity, but once it hits a tree branch it loses a lot of its power. That's not really that helpful in reality.
At the ranges SS is played at, it matters more because 5.56 would very clearly outperform 7.62 on direct hits. What I would like to see in the game is implementation of realistic penetration for both rounds. A 7.62 should go through most everything that isn't a very solid wall. If you see an enemy through a window of some plaster building and they duck below it, you blasting that wall with a battle rifle should get you a kill. If that's how it worked, I would consider the rounds balanced as they have specific strengths.
Not being very knowledgeable about these sorts of details, I find this very interesting! Perhaps another great opportunity to further differentiate between weapons as you suggest.