Be careful, stiffer rear springs with soft fronts could make the car oversteer, or like NASCAR people say "loose".
I know people say it has a lot of understeer so using stiffer rears may get the balance closer to neutral or beyond towards oversteer, it may be more than stability control can handle.
I would not recomend you guys try that unless you have autocross, track days, or racing experience.
So Babybenz, what do you think about the handling?