Such fleets sound very effective, I dont doubt they can do it easily.
Also, all tactical consoles got diminishing returns, and I dont see why the fleet ones would not have it.
I would say they need to be at least as good as the Mark XII purples, since they are mark XII ultra purple rare.
So they need to be better than the mark XII purples. Which means they either need to be 30% + some other bonus, or bigger than 30% damage boost.
they don't have diminishing returns, they just only buff the base damage, not the damage after skills and power levels. This is why you can put up to 5 of them in, unlike engineering armor consoles, which are subject to returns, and after the 2nd the thirds all but not worth it, and a 4th or 5th is a total waste of slots.