Yeah... And I know its rare, but I still want FF+GRG to work and be worth while. I want to keep the AM cost high, so I will say again the solution to the Kol is a damage increase.
If want something other than that, I would say something that degrades the shield mitigation cap of the target by 8/16/24% for say, 10/12/14 seconds. That would make the Kol suddenly worth while. Such a devastating attack could mark an enemy ship for death. Forget the damage boost or making AF passive. That sort of an ability late game would be a death sentence. Essentially doubling the damage you can deal to an enemy capital or starbase. That is huge boost.
And in all reality, firing a slug that size would disrupt the onboard computers attempting to neutralize damage, and you would also be destroying much of the shield so suddenly the computer would have a fraction of its shield mass to work with. That would degrade mitigation quite a bit.
Also, if anyone says you can't do this as there is no buff, there is a buff that you can apply. It is in the research tab. The Advent can research higher mitigation via this tool. Also, in a mod I am working on, there is a ship called the Eyron which specializes in shields. Whether its draining them, destroying them, or sacrificing its own to blast another ship out of existence (the level 6 ability btw), it still deals with shields. Its other ability is a passive similar to the Halcyon's AEA. Only in this case it amplifies shield mitigation slightly. Not much, but a bit. So my point is, the devs would not have to create an entirely new buff. And honestly, if they don't put something in in the next patch, I'm sticking this in my mod unless I see an idea I like more.