If the feat we give the archer is a pure Power Attack analogue, you are correct. This is why I like Nifft's solution so much- it requires the archer to literally give up attacks to power it up. Thus, the archer trades the ability to do damage quicker for the ability to do more damage at once. The melee attacker still gets the PA damage on every successful hit, and will still come out ahead in the final analysis; the archer, however, now has the potential to deal enough damage in one shot to stay on par with the Power Attacker's per-attack total.
