I`d defo not fit the harness until you`ve got a proper seat in dude
Try some toyo T1R`s for tyres, thats what we race on and they car easy keep up with cars on slicks/888`s and last really well. They have great grip in the wet or dry! LSD is a great idea but the 1.8l brakes aren`t needed really, some green stuf pads for the road and some yellow stuff pads for track use is more than enough.