FLAK or Anti-Fighter Turrets are a great idea, and cool as hell to see firing, but really, they're about worthless. Fighters, even the shortest ranged ones, can engage targets from at least four or five klicks away. These turrets only have a about a half klick range.
To make them useful, perhaps you can extend the range out to say ten klicks, but only let them fire on fighters only.
I agree that FLAK cannons are and should be a short ranged weapon, however, we are talking about space here; no fighter in it's right mind will ever get close enough to it's target for the FLAK system to be a threat.
Just my opinion, for what it's worth.