Always improve the driver and safety first. The harnesses combine the two. It will keep you in the seat, so you are safer and you avoid getting so tired holding yourself in. When you are less tired, you make fewer mistakes.
After that, for speed, improve the contact patches. Anything that contacts something else - Tires to the pavement, brakes to the rotors, seat (to your butt), harnesses, steering wheel, pedals...and so on.