Wording on JP wiki suggests that every weapon is rolled separately. I have only one Gluttony Gun FLB for now, so I can't check it myself. Sorry.
That's how it is for reductions (damage cuts, atk/def debuffs).
For increases the more you get the less relative increase is.
As for Stinger example:
1. You have 50% chance to deal 120% damage, your average damage is 110%. Realtive increase is 10% damage.
2. You have 100% to deal 120% damage, obviously your average damage is 120%. Relative increase to point 1. is 120/110 - 1 ~ 9.1%.
In general same holds true for all increases (assault, defense, elemental attack).