So, I use a Support Gunner mod for my Soldier (solo play) because I enjoy the unlimited ammo it gives me. The idea just occured to me however, that I could possibly use a Leader Mod (that gives extra % Exp) just for turnins. I would equip the extra exp mod, turn in the quest, and then re-equip my Support mod. Would this work?
No - the Leader Mod does not influence experience gained from handing in quests, as discussed on the GearBox forum. The experience gained from this bonus is pretty much limited to experience from killing enemies.