Depends on what your using the car for, but I wouldnt ever go with heavier springs. 8k/6k is already way more than enough. Get a bigger front sway bar if you need more roll stiffness. The only thing heavier springs is going to do is make it ride worse and rattle your car apart.

And this isnt pointed at you specifically, but unless the car is going to run some serious tire like DOT-R's or Slicks you dont need super stiff springs. All stiffer springs is going to do is transfer more load on the tire, and most road tires dont have enough sidewall strength to cope with it. So thats going to have some adverse affects on its handling characteristics. Its also going to make chassis flex more apparent.