I think that the in the specific case of using against interceptor class vessels, the fed plasma emitter is overpowered.
It is fixed, unavoidable damage that will kill an interceptor if only slight other damage is applied on top.
It completely negates the primary defence mechanism of the interceptor (avoid damage) because it can not be dodged nor be deflected.
It does nearly enough damage to kill unless you have a heal ready, meaning in a fight interceptor against interceptor the one using the plasma emitter will only have to do some 1/10th of the damage the other guy has to do, severely one-siding the fight.
The ability is OK against assault ships and frigates because of their much larger HP pool making it easier for those ship to mitigate the damage. Also it’s perfectly OK to give the fed interceptor a leg up in damage against a harpy.
As a solution I’d suggest first making the ability into a non-homing shot, so player skill is asked for when trying to target interceptors (and assault ships to a certain degree).