Well, what about this?
1/2 Aim is a Max of +1d10 Bonus damage
Full Aim is a Max of +2d10 Bonus damage
Each 1d10 per 2 DoS as normal, and you need to make a called shot as well.
It seems to be the best mix of reducing the nonstop sniper of death problem, while letting skilled players get the bonus as normal. Afterall, a normal guardsman could pull off a good shot at times, but a trained guard marksman can do it reliably
Edit: Something one of my GMs suggested for making full auto more realistic is a +0 to hit, otherwise, leave as is.