I would limit bonuses to damage with the ranged weapon to 30 ft to be consistent with other ranged damage. Also limit weapon specialization to +2 (to match the feat) - note that a fighter can't get greater weapon specialization (+4 to damage until 12th level), also don't give greater weapon focus unless you first grant weapon focus (again for consistency with normal feat progression).
I might structure the concentrated fire concept to follow the power attack style instead (trade off BAB for damage bonus).
As far as the actual numerical value - well check out some of the feats in PHB II for reference. Ranged Weapon Mastery for one (Prereq +8 BAB, prof, weapon focus and weapon specialization with chosen weapon type) - then gain +2 on attacks and damge with any ranged weapon that does that type of damage.
I think I would find a way to work in the following feats as class abilities for this type of character: far shot, point blank shot, precise shot, etc.
I might structure the concentrated fire concept to follow the power attack style instead (trade off BAB for damage bonus).
As far as the actual numerical value - well check out some of the feats in PHB II for reference. Ranged Weapon Mastery for one (Prereq +8 BAB, prof, weapon focus and weapon specialization with chosen weapon type) - then gain +2 on attacks and damge with any ranged weapon that does that type of damage.
I think I would find a way to work in the following feats as class abilities for this type of character: far shot, point blank shot, precise shot, etc.