I went through their downforce guide and it is very helpful, I got convinced into getting only front splitter and rear wing for my planned aero, as they are the most simple and effective way to gain more/cleaner downforce.
and here is my car weight, very close to your car weight, but I think with half fuel tank.
Also, about spring rates, this is something I noticed when I got my mca reds, there was a big difference in front and rear, 21k/7k, when I had my rear height higher than fronts by around an inch, the car was nearly neutral, but when I tried with lower rear, around 0.5 inch higher, the car understeered at exit but my turn in was great. Many factors could affect understeer/oversteer, it could be adjusted easily unless it is undrivable.