I haven't tested them yet because my own ones still aren't assembled yet, but I've seen them in a recent Steve's batrep against Space Wolves and I can tell you they have potential inside a TS army. They are the only Chaos terminators that aren't a character which can reach a 3++ save and getting Cyclone Missile Launcher and Assault Cannon equivalents. And like you said, they give you a decent 2 wounds, ML2 psyker and the AP3 Combi-Bolters are pretty nice.
However, are they worth it in a BL army? I'm going toward no as an answer. This is because they won't benefit from the Benediction of Tzeentch from the TS legion tactics, so they will be stuck with a 4++ save. Morover, like you said, except their rending Soulreaper Cannon, they are devoid of AP2 except if you are lucky and you roll Iron Arms on Biomancy for your single non Tzeentch power on the sorcerer. Also, there is no way to put them in the BL Speartip detachment, so they won't benefit from rolling for reserve on turn 1 and won't get objective secured like the BL normal Chaos Terminators. So, in the end, except if you want to make a fluffy Black Legion's Sons of the Cyclop warband (it's the only BL warband entirely dedicated to Tzeentch), I don't think they will be worth it, as much as it pains me to say that. They are absolutely amazing models though. A better choice for a BL CAD, if you want to add some more psychic might, would be to field an Exalted Sorcerer. This is a great model in itself having +1 BS (very important), +1 Initiative, +1 A and more importantly, a nice once per game S9 AP2, Lance, Small Blast attack and more importantly, access to Divination. Your Chosen or heavy support could use to reroll to hit and/or ignoring cover, isn't it?
Of course, you could add a side CAD to your main BL detachment though. In this case, the Scarab Occult Terminators make more sense.