I think difficulty selection doesn't change the difficulty itself. It just changes the spawn rates of the enemies within the mission as far as I could tell. As you got to level 8 and geared up, shots seemed to do similar (possibly marginally more) damage than they did at level 2 with the weaker gear. This is likely why damage per second/damage per bullet people are requesting isn't ever listed as a numerical value, since it works more like a % of health per shot rather than having a predetermined value.