You mentioned shacktactical as an example of high fps in MP, just a quote from dslyecxi from one of his videos: "Arma 3 performs very poorly in multiplayer on the Altis terrain. It's just not fun to play right now in ST MP at playercounts even as "low" as 50-60 people. We'll need to see some performance improvements before we really start playing it seriously."
So yeah, MP performance have not only not improved, it is even worse than in Arma 2.