Cheers, what you put makes a lot of sense. I have tried using the agoniser on the syren, but it never seems to get that 4+ wound; and as Lilith runs with that group, and she will normally get 13/12 attacks that all ignore armour saves, I find that I like having the blast pistol when Im faced with some armour before I charge it, (twice ive blown up a landraider with the pistol!).
The Sybarites and Dracons I agree completely with, don't really need them. At the moment I do run all 10 Kabalite trueborns in a Raider-will lose the splinter racks, and if they get within 18" everything dies. Saying that though I do fancy some venoms and I can see the upside of using 2 5 man squads in 2 venoms.
The Talos though, I've found it achieves 2 things when I play (usually against ravenwing or GK) It really absorbs their fire. They shoot everything at it, which really helps my raiders get into range of them. When I played without it, all that heavy weapon power just ripped my raiders to pieces and I was left with everything footslogging. Also I find, that if you get it into a squad of normal marines or even assault marines (bearing in mind they need 5+ to hit and 6+ to wound and usually will get feel no pain) it A) holds that unit up/bogs them down B) Often kills that unit (depending on random attacks) or wittles them down enough to make them easy pickings for the rest.
I do like the Venom idea though and may look at that in the future.