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OK, I'mplaying in a 40K doubles tournament. My partner is playing WH and is taking 2 Inquisitors with Gun Servitors as a basis for his list. My list is:
Elite Inquisitor with Psycannon
8 ISTs with 2 Plasma Guns in Chimera (Multilaser, Extra Armour, Smoke, Heavy Bolter)
9 PA Grey Knights (1 Psycannon, Justicar with Melta Bombs)
May not be 100% accurate as I dont have the list in front of me,,,
WH shooty retinues aren't as powerful as DH ones - you lose the ability to take that Plasma Cannon. And you can't purchase Heirophants, who give a handy +1Ld to the Elite Inq.
If your friend is covering shooting so well, why take those IST at all? You'd be better off with two fairly large GK squads, no special weapons in either of them.