Something to consider is that, AFAICT, IR lasers have exactly the same power and range as plasma carronades, and cost half as much. The advantage of the carronades is that it's faster to research any given caliber, but for something you're mounting on an FAC, you won't want a very big weapon anyway. Plus, lasers have the smaller/slower firing techs, making them even more suited for the job.
Really, I don't know what carronades are supposed to be good for.