I don't think the need for super fast followups is as great as penetration of each round. Especially in the urban environment which I live.
Not me, I am the other away around. I have shot both the M14 (M1A), the M16 (AR15) as well as the M1 Garand in competition. For me, I know that with my .223, I can very effectively place my shots at 500 yrds with iron sights against a stationary target. At 300 yrds, I can effectively place my shots with rapid fire. Moving targets get fairly easy at 200 yrds.
I have been to the training centers and have done the penetration tests with 5.56, the 7.62x51mm and the 7.62x39mm, both on brick and cinder block faced "structures" as well as against automobiles. All rounds penetrated the building material effectively, although the 5.56 did need one or two extra rounds to take down the barrier enough for the follow-on shots to be a lethal to however was behind the barrier. As for the automobile test, none of the rounds had trouble penetrating and destroying the "occupants" inside. The only safe areas that offered even a modicum of cover was the engine block and the axle areas.
So, for me, in an urban environment...and NOT the foot hills of Montana, the 223 is all I will need.