At T1, static AA is basically 3 mobile clumped together, at least mass and DPS wise.
Average T1 AA comparison, static vs mobile
- Mass: 150 vs 55
DPS: 68 vs 27
DPS/Mass: 0.45 vs 0.49
At T3, static AA is much more efficient in exchange for the loss of mobility.
Average T3 AA comparison, static vs mobile
- Mass: 800 vs 840
DPS: 333 vs 181
DPS/mass: 0.42 vs 0.21
At T2 though, mobile flak absolutely wipes the floor with its static counterpart for no apparent reason.
Average T2 AA comparison, static vs mobile
- Mass: 400 vs 160
DPS: 150 vs 146
DPS/Mass: 0.38 vs 0.91
The only reason you'd ever build static instead of mobile flak is because you're panicking at an incoming snipe. Otherwise, mobile is just as effective (if not more because it can dodge and follow air) for only 40% the cost.
I'm not sure if mobile flak needs a nerf, or static flak needs a buff, but the (lack of) difference between the two doesn't make any sense.