Sacrificing damage for accuracy, as long as the damage isn't reduced for over 20% i'm fine.
I am the same way, but my limit is usually based on how many more shots it takes to kill after the reduced damage. Decreaseing damage so that it takes more bodyshots to kill is fine, but when a weapon that used to take a certain number of headshots to kill starts taking another one, or even two, it starts to get annoying. So I'd be willing to take a bigger hit on damage on a weapon that does 33% damage on a headshot than one that does 34%. But then again, this is a lot more subjective with brink than other games, as no one is going to have the same amount of HP in this game.
When this game comes out, i will most likely use a SMG gun. What modification is the most useful to an SMG gun??
The two main schools of thought are aim, and spray. So you might make your guns as accurate as you can and aim every shot, or stuff your gun full of ammo and just hold the trigger down, as well as increasing recoil stabilty to put as many bullets as you can on target. Aiming will tend to be better mid and long range, spraying will tend to win out short range.
But you're not even forced to fit into these two stereotypes. You might just increase fire rate and damage, and ignore accuracy/stability. In that case, you might miss a lot, but the bullets that do hit are gonna hurt.
If gun customization is truly balanced, it is going to be an expression of your own personal playstyle; what works best for you, not whats best overall.