I love you so much for this. I was looking at implementing this into UE4 for a personal project of mine as well. Have you looked the planning queue at all? I know the reason why FEAR had so few AI on screen at a time was because of how hard they were hitting their planner, once every couple of seconds or so. They also had to culling of their GOAP actors as well, so the Rats at the beginning of the first level were still planning even if the player was in another part of the level. Why I mentioned this was that because the more actions the AI could put into their plan queue the faster the system ran with more AI at the same time.
Thanks for doing a lot of legwork here, I will definitely keep an eye on it!