The full auto does not down in 2 headshots nor 1 headshot + 2 body shots and never has.
I don't know about you, but I just played a match where I downed an enemy with 1 headshot and 2 body shots with the FAR. Then again, it was a public match so all sorts of variables could have been in play, even though to me I saw him get hit once with my FAR in the head, and two more times in the body.
If you're up for it, we can test it out in a private match with just the two of us. One of us could just stand and get shot at to really make sure what the exact amount of bullets need to hit.