Well have used them in 5 2K pt games and they have only done two slightly significant things. In the 1st 3 games they were killed without doing anything noteworthy. Game 4. Turn 2 put wounds on a Fex, and then got shot up (poor save rolls on my end) one was left (w/ melta) turn 3 he managed to be falling back past a Mawloc and put a wound on it; he was killed in Nid shooting round by a stray template. Game 5 Turn 3 used the infiltrators option. They shot up a Genestealer and inflicted casualties on all shots. Then a Fex shot them up and what was left of the Genestealers assaulted them and killed them.
Sergeant - Boltgun
Plasma
Melta
4 Hotshot Lasguns
Is what they are using.
The 1st 3 games it should be noted used them in deep strike and the last two as infiltrators. And no, unless they scattered, i did not intentionally deep strike them in front of every bug w/ gun.
The last 2 games I have seen some more production from them as infiltrators, but w/ the short shooting range i am most likely to be in range to be assaulted by anything w/ fleet.

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In the event that you may not know if your target is down - Double Tap Em!