I agree that +0 archer items should be buffed. I am not sure how to weaken +3s at all, since I've never done archer. I don't think simply nerfing +3 item stats will do much help.
Make archer score more equivalent to what melee have, or give bonus points for headshots.
Change score area from near an enemy taking damage to the area near teammates who are gaining points (incl. range). Enemies getting teamhit next to you should give you points, just for hilarity effect. People in clusters will still get points, just not as many free points as before. These have been suggested by others for a long time, and they sound like pretty good ideas that don't seem too hard to implement.