The cultist knife if just a bit of extra kit to go with the single arm holding shotgun/autogun if you do not want to use the banner. Its got no in game rules for Kill Team and is just there to look cool (mileage on looking cool may vary).
TBH the genestealer cult needs another box to get up to that 100 point starting list but what you have is fairly solid. I also would favor the shotguns as Kill Team is very close range and they let you be slightly more mobile than the autoguns.
Thanks, that's what I thought. Just wanted confirmation. I actually have some unpainted Genestealers from Space Hulk (4th) that I'm going to paint up to stand in as purebreds for the 100 points.
I have Rogue Trader and Blackstone Fortress as well, so I think I'll have enough to keep me busy until my wallet recovers from my GW splurge