The suspension needs some movement.

And you'd be surprised with no suspension how the smallest pothole will send you off the track.

Stiff shocks and soft springs are good. But having no suspension, or having the car bottom out is the worst thing.
Most people have never put good quality shockers in their cars, and "go overboard" with ideas to improve things. Start with good quality HD sports shocks and VR rated or ZR rated 50-60 series tires. Cheap tires means cheap handling.

The extra leaf is a great idea. I did reverse eye extra leaf springs on the rear of my old 1200/CA18DET sedan..it felt much better to drive. But with more than 120kw at the wheels, you may need to look at installing tramp rods, slapper bars or re-engineer the rear end like I'm doing.

Chris