As you requested, Jasper, here are the possible bioweapon choices for the Carnifex and their pros/cons:
Firstly ignore the Rending Claws or two sets of Scything Talons altogether. Carnifexes are too slow to be purely for CC.
Fleshborer: Pros: Eh,...it's a ranged weapon...
Cons: Won't benefit from strength bonus
Minimal range
No AP
Spinefists: Pros: Decent AP
Cons: Minimal range
Devourer: Pros: Multiple shots
Some AP
Cons: Minimal range
Deathspitter: Pros: Moderate range
Small Blast
Decent AP
Cons: Won't benefit from strength bonus
Barbed Strangler: Pros: Good range
Ordance blast
Decent AP
Cons: None, really
Venom Cannon: Pros: Great range
Good AP
Multiple shots
Cons: Won't benefit from strength bonus
All in all, the best combination is Scything Talons and a Barbed Strangler. Add Bio-plasma. Stir and simmer.
Happy Trails.