I have both Gringo and Union rifles upgraded.
I hear a lot about sector bonuses but I don't think they have that much impact to be honest. First you have to be in the same sector as many people, this is mainly in attacks where you hold in large sectors and shooting towers. When you do so most people miss, lets say 60% which I think is low. Then you are only buffing the 40% that hit. Considering that in that large sector there is definitely minimum someone with damage boosting gear,lets say Murrieta which gives 150 damage bonus. This means you are only increasing the bonus with 140, and only for those 40% that hit.If you have 20 people in your sector, 8 would hit and you would give them and extra 140, we are talking just above 1k more damage a round. If there are people with Union the difference would be much less.
To say you increase 9k a round seems like bad maths to me.
In defence you are almost never in a sector with more than 8 people (tower).
Personally when I go LD I use Union weapons as I make much more damage.