Agree with the Count.
Think of it the other way round; if a Haemonculus has joined a squad of Kabalites or Wyches he is rolling with majority Toughness of 3. If instant death applied on the majority, he'd be removed by a single S6 wound.
So while it would be a hilarious way to get rid of high toughness characters and Eldar Guardian weapons platforms, common sense prevails and a model's own toughness is used for instant death purposes.
The Dark Eldar and Dissynergy.
3d printed Dark Eldar bits on Shapeways.