I noticed front grip increased a lot more than the rear when I use racing tires. Thus car turn in more sharp but rear seems slide out. So it makes car more twitch and feels unstable. And some cars are affected more than others. But same time, I think braking distance is shorter and if you put more weight on rear by opening up throttle earlier, car actually can corner faster. Some of cars, if I tune with racing tire, it becomes under steering pig when I'm using SS.
I don't know it is realistic behavior or not when you put different tires. And I don't know why some cars take different tire well while others changing characteristic. But it seems this is how it is on GT6