All cars run great on R5 (super soft) tires, the good tuners use sports tires (medium/soft) for tuning, so when you switch from sports to racing tires after tuning the car on sports tires it will handle much better then went tuning it with racing tires.
Try it out and you will know what I'm talking about.
Because if you tune it with racing tires when you switch to sports it will handle horribly.
My goal is to make every setup with out any downforce if possible.