Thanks for the clarification on the cartridge length. I always forget which is which.
And while you are correct about long range effectiveness vs. the 5.56, they still penetrate pretty well, and I would submit that the GI weapons don't need to be effective any further out than the M16/AR15 is. You're talking sniper rounds/weapons, which are more likely to be chambered in the larger rounds you mentioned above (or a completely different round of a similar size). Additionally, I don't know if I like the idea of the larger round being issued to LEOs in an urban setting. Seems you'd want the lighter round for safety's sake.
My argument for the heavier round comes from the LA shootout with the two perps wearing body armor and firing AKs. It only ended when one of them shot himself and the other had his legs shot out from under him. Even though the cops borrowed some AR-15s from a local gun store, leaving a couple of M1-As on the rack, they didn't seem to be able to penetrate the perps' body armor at the range they were shooting. An M1-A round (7.62x51mm) would have gone right through that armor and immediately stopped the subject.