yeah, fx is a heavy beast and it will want to understeer like crazy, adding a lot of rear spring rate and stiffening up the rear a lot can help a lot, lowering and stiffening it up in general will help too, ideally you want to get the rear of the fx to be much more loose traction wise than it is relative to the front... not so easy but a set of coilovers can make this much easier assuming you can take care of alignment issues too. when you got it right then instead of plowing through the turns you'll just have to worry about pushing it too hard and rolling it or spinning the ass out

when you can get it to spin the ass out you're on the right track
also if you are going to want to do this several times a season then separate track wheels is a good idea, you will def chew up your tires