Mercenary | Commando DPS Compendium (2.0 Updated)

[Fungus]

Yes.

Damage for Ranged (not tech) attacks uses your weapon's damage values.

In my experience, for Commandoes 1 point of average Mainhand-Damage is worth approximately as much as 1.4 Aim. A little more for Assault Specialist and a little less for Gunnery.
I believe it should be about the same for Mercs if you replace both barrels.
This alone should be sufficient to balance the loss of Aim for expertise, but additionally you get more Tech-Power.