Just bought an E30 325i 1988 and it has new front and rear koni sport adjustable shocks with stock springs. I've been looking for stiff springs in Europe like H&R race or stiffer but haven't found an after market option yet. But today i found out BC sells many springs in many different spring ratios Springs (bc-racing.co.uk). But I need some help to ensure I don't fuk up with the set up.

So fronts should be easy, BC offers straight springs with flat ends (which should fit the spring cup). The diameter should be the same as the stock spring but I don't know the length I should choose. I know i want a 6mm drop but not sure if I should just rely on hooks law to calculate length (F=k*x) considering each corner of the car should weight 300kg.

For the rear it's a little more complicated, BC also offers barrel springs like the OEMs. I thought barrel springs where always progressive but they just give one spring rate ( anyone knows if they are non progressive or if they squish on the middle like HRs ?). Another option is to go for straight springs for the rears but not sure if that's a good idea.

Spring rates I have in mind: 8kg/mm(=450lb/inch) front
9kg/mm(=500lb/inch) rear

-What do you think of these spring rates?
-Does anyone know how to calculate the length of the springs I should choose?
-Will this stiffness wear down suspension parts that I should reinforce ?
-Is there any chance the rear spring can pop out due to the high spring rate and shorter length? How can I solve this?
-Will the konis be strong enough for these spring rates?

I know a lot of EU members have asked for stiffer springs so I hope this can help them too.

Any advice is welcome

btw no coilovers please