I was thinking, tell me if you think is to complex, that you remove ranges from weapons and replaced it with range bands. Example a bolter would have a 6" range band, and a Tau pulse Rifle an 8" range band. Then the BS would drop at each range band increment. IE a Marine would have a BS 4 up to 6", 3 up to 12", 2 up to 18" ect. Maybe this would offer the ability to raise the marines BS to the fluffed level and raise the level of others to account for the individual fluff. Of course Tau aren't as good as a marine but they are better than a standard guardsman.
A side note I would make pistol weaponshave a special 1" +1 BS rule with a 3" range band making a Marine BS5 at 1", 4 at 3", 3 at 6", etc..
Assault weapons would get a similar rule at 3" instead of the 1".