You can "probably" just math this out. Using FB as an example, you get two poisons and poison sensitivity to boost your existing poisons.
Both Fayne and FB do two-turn duration poisons, and let's leave the blue mastery extending it out of the calculus for now, for simplicity. Fayne's poisons will get boosted for both turns, while FB's will only get boosted for one turn.
Without FB, Fayne will get 50k * 2 * 2 = 200k damage every three turns from poisons, for a total of 66.6K damage per turn average.
With FB, Fayne will get 62.5k * 2 * 2 = 250k damage every three turns from poisons, for a total of 83.33K damage per turn average, while FB will get 62.5K * 2 + 50k * 2 = 225K, or 75K per turn average.
So put another way, FB's contribution is going to be basically negligable face damage but three WM procs for about 60k apiece, + 75K from her poisons, + ~16.66K from her extra damage contributed to Fayne, for a total of about 150K damage per turn average.
If Belanor can do more than 150K damage per turn average, then you run him. Same goes for Konstantin.