You don't need to get 8 headshots with the Carbine, nor do you need 5 headshots with the BR. You just need to place that many shots on the opponent's body to pop their shields, which is when you go for 1 headshot.
Since the Carbine has a much faster RoF, 7 shots to the body and 1 to the head shouldn't be that big of a deal compared to 4 and 1 with the BR; actually, if you land every shot perfectly, the Carbine has a faster kill time than the BR.
It's a pro-weapon that only the top 1% of Halo 4 players should use. I'm pretty good with it, and only getting better.