A spring is basically a spring. Springs from quality suppliers, like Eibach and H&R, are more even (get two and they will perform the same) which cannot be said about the real cheapos. Some manufacturers, like Öhlins, treat their springs to all kinds of stuff (heat, peening, nitrate or whatever) making them better still but to what price?
Personally I don't like progressive springs but that's me.
I'd go for as stiff springs as my shocks could handle. The harshness of a ride is far more related to the shocks than the spring rate and it is a common mistake to put on too weak springs and then compensating with setting up the dampers hard.
Factory springs are often very weak in order to provide maximum comfort while masking the poor performance of the shocks. The average price for a Volvo shock is about 10 USD and I suspect that goes for most car manufacturers (Porsche hopefully not included). Hmmm, I wonder why my kidneys are bleeding after lowering my car 1,5" with the SportySprings performance spring package for 29.99?
Most springs-only kits shoot for the stock spring rate time x about 1,5 or as most x 2. Having done no research about this for our cars I would suspect it to be about the same. A decent shock like the yellow should be able to handle much more than that, perhaps at least x 3, and that's what you should go for. Then set the shocks up soft for street.
For comparison, I replaced my front 600 springs with 800 since I thought of them being to soft and that I found myself running in the upper portion of the shock adjustment range all the time. The ride quality with the 600's is imo street car quality and not at all harsh as you'd might expect. I haven't tried the 800's yet so can't really say anything about them.
