I think we are just talking about the same thing since what makes it insane on a 2 pop unit is that you can make more of them. if they get a pop reduction but the person still only makes the same amount of unit and nothing else then nothing really changes.
Treaty player here. Your math is a bit off. Legionarios and Redcoats are equal (without thin red line) and their attack stat comes out to 58 damage. A standard base imperial musket deals about 51 damage. (Extra context, USA regulars deal 57 damage outside the flag and 63 inside it). Soldados deal 44 dmg per pop outside the flag which is a bit more than a fully upgraded guard redcoat (around 40 damage)