Yea, highly focused multipliers doesn't really on a short ranged, immobile ship type.
If the flak turret could have its multiplier values knocked down by 1 or 2, but gain even more in base DPS in return, that would be cool. I wouldn't even mind it losing some of its newly gotten cap HP in return, as long as it remains the highest cap HP of the turrets.
I can't tell how large the burst radius is (for some reason, the "explosion" doesn't show on my machine), but it may be a bit too small. Or go with what chemical_art said, sacrifice some of its AOE type effects in return for more direct target DPS.
And yes, the X targets within Y range thing for any unit that uses it should be made clear on the description or command card or whatever somehow.