The easy answer is this - tactical consoles do not suffer from diminishing returns. You can test that they add the same amount of damage each time you add a console, by unequipping all your tac consoles in system space, and checking the numbers in the tooltip when you mouseover the weapons in your ability bar.
I just did this with my advanced escort (4 tac slots) to make sure I was not spreading bad information, here are the relevant numbers.
All tac consoles were blue quality plasma infuser consoles (+26.2%)
Weapon checked was purple quality plasma mk XI DHC with base damage 565(376.7)
It is quite clear that each console adds the same amount, but it is unclear exactly how this amount scales in relation to skill points,power levels and base weapon damage - and TBH I'm not going to spend a long time reverse engineering the formula, when the important thing that people need to know is....
Tactical consoles do not suffer from diminishing returns.
The actual percentile increase per console is smaller than the amount stated in the console's stats.
So with this information is it worth it to use a launcher or just stick with 4 cannons when running a 5 slop tactical advace fleet escort? mk xii relays very rare are 30% x 5 is 150% more damage which should put a dual heavy cannon mk xii very rare doing more damage then a mk xii very rare laucher.