I have played a lot of Borderlands, but just for fun and relaxation, rather than trying to technically pull it apart. I generally just play solo using whatever I find along the way. After playing all the characters (several times) I got fed up with the same playing style and decided to try different builds (finally).
As I begin to think through skill tree choices, I wondered how skills with the same boost interact. For example Axton, Impact and Onslaught both boost Gun Damage. Maybe its a bad example but for the sake of the argument is the result additive to Gun Damage ?