You're right, but... Take a mantis guard. For 3 AP more I have: +1 in CC,+1 in LD, +3 in armor, only one point less in def, and as you said, 2 points less in damage, which are more than compensated by the group attack, and can be delivered at a distance having a polearm, and I have to remember you that being the goblins size 1, they can attack a normal humanoid till eight on the same, so having a polearm is very nice. More than this, you can add a novice shaman to the warband.
There's no reason why I should prefer the headhunters.
I ask for an official adjustment. I can assure that, even elevating the cost and making it equal to the mantis guard (36 AP), but adding the berserk ability, I'd field them with great pleasure. This would add a real "boost" in the strategic value of the band.
Please let me know if you agree with the idea.