Flak are too short ranged.
This seems to be true. The range of a Flak turret (5000) is less than half the range of a Needler or Laser turret (10500, 12000). It's about 60% the range of a Lightning turret (8000), despite the fact that the Lightning 'range' is pure AOE and can hit up to 200 units in that 8000 radius, while the Flak can only hit 5-7 units in a 500 radius at a maximum distance of 5000.
Of course, hitting 5 targets, a Flak can do 13,000 DPS before bonuses, while a Lightning turret doing max damage (40-200 targets) can only do 1,400 DPS, or almost 90% less.
So, it may be worth it to drop off a little range in exchange for 9 times as much damage.
(Or maybe the Lightning turret needs a little buff? 1,400 DPS is 1/2-1/3 other turrets, except Spider - which has infinite range, Engine Damage, Armor Piercing, and bigger multipliers).
Their point isn't to be better than FFs ... but to provide a way of getting more FFs and also have a tactical wrinkle.
And this is what they get used for in my games. A FF to protect something is better than nothing. Even if HFFs were purely worse, they'd still be useful just to have more FFs.
That still puts them in front, covered by a force field, where the plasma siege can then peg the forcefield with impunity. Which it will probably do.
Edit:
Which *math* does 1,250,000 to everything under the FF (over the course of the bombardment that it takes to kill the FF). Flaks have *gasp* 1,240,000 health. Oops.
It takes 200 seconds for a Plasma Siege Starship to kill a Mk I Forcefield. With the same last shot that killed the FF, it would also kill up to 25 Flak turrets that were somehow under the FF the entire time.
During those 200 seconds, each Flak turret would do 390,000 damage to the Plasma Siege. So it takes 26 Flak turrets to be able to kill a Plasma Siege starship before it kills the FF they're under. A full cap of Flak (48) vs a full cap of Plasma Siege starships (2) is an easy win for the Flak, thanks to Flak's AOE.
Considering that Flak are designed to murderize low-hp swarmer units, not duke it out with slow artillery starships, this hardly seems unreasonable.