Another Word Bearers player here, I have used possessed as both their 40K and 30K equivalents. But seeing as prefer my possessed ADB style, I tend to use the Crimson Slaughter supplement for mine. It may have just been me being very lucky, but a unit of 5 possessed with mark of tzeentch worked very well for me, with a 4+ (sometimes 2+) invun, they can be pretty survivable. And with the new Crimson Slaughter formations, feel no pain at no extra cost. As I said, I'm not sure if I'm just very lucky, but each game iv'e played, the possessed at the very least have made back their points.