Well, last I recall from live, Dual Pistols came in dead last in terms of DPS. That was with the old Snipe change. I'd say the new snipe change likely continues the trend.
Agree with Piercing Rounds always applying the -resist.
Maybe that would be the change that actually helps the sets DPS.
Though, I may have preferred something different with the ammo types.
Keep the DPS the same on them all. Different DoT for the ammos. Lethal, Cold, Toxic, Fire DoTs, with each set giving different secondary effects. Fire w/ -Resist. Ice w/ -Spd/Rch. Toxic w/ -DMG. Lethal w/ KD.
Fire ammo would still do more damage due to the -Resist, but with good damage on the other ammo types, could be a better reason to switch ammos based on some enemies who may actually have a weakness to them.