Force shell is a deeply PvP-oriented modification. It is used to shake off aim of your opponents while you are charging them, and counter FA-OFF pilots. I get the feeling that your vessel is not a PvP one. Neither you will get any noticeable effect with only one cannon.
If you really want to use them in PvE, slap a cannon into each slot and meme NPCs into 'roids. It can even be super effective when you get a knack for it. Just make sure to adjust projectile speeds into one reticle via longrange, as different sizes have different speeds. And use fixed cannons, or you will never hit anything.
Small hardpoints on a Vette basically scream rails.
Also, I suggest replacing turreted beam with a gimbal one, as turreted thermal vent might melt you down if NPC have two chaff modules.