i do basically the same thing here...this does 2 things..I run mine as 3 x combi-melta and a heavy flamer in a drop pod, then aim to annihilate something beefy when they drop in. Otherwise i'd just go standard loadout with the special issue ammo.
With regards to the 2+ armour save, the poison rounds work out pretty well if you have rapid fire range.
I sometimes run a powerfist on the sgt as well - with their extra attacks and special ammo overwatch they can be quite intimidating to charge.
1: can mess with the opponent deployment..as theyll need to protect their valuable target..
2: can drop in kill something expensive and then if still around can cause harassment(tho expensive to throw away)