this is definitely NOT guess work.
i've spent about 12+ hours and 2-3 million (mod swapping) testing both Merc/Pyro and VG/Assasult, multiple specs.
TORParse breaks down everything. that's not saying i have the best rotation for the class/spec, but i do what i would do in actual combat. i'm sure there is some room for improvement.
IMO Pyro Merc does a bit more than what you did with your vanguard, except at 30m range.
Technically there's a small benefit to being at rocket punch range as a merc pyro but it's not worth being there for in an ops fight.
Also in an ops fight you have someone applying an armour debuff which adds about 8% more damage to the kinetic and energy damage a pyro merc does. An arsenal merc doesn't get a damage increase in an ops like that.