Progressive rear springs for Prius?

Your description of progressive springs is a bit confusing. The term"progressive" alone describes the springs. Instead of linear springs, which compress proportionally to the load, progressive springs start out with lower resistance to compression at first, often lower than factory, to provide a very smooth ride, and then compress proportionately as the load increases, thus resisting compression more and more as the force increases. This proportional increase in compression provided great resistance to tipping of the car and improves handling on rough roads. And yes, B & G makes progressive springs for the Prius, as does Eibach. Eibach are a much better deal at about $250.00 a set. With any progressive springs, good gas shocks should replace the factory oil shocks, and, since the car is lowered an inch or so, the Eibach front and rear alignment kits should be purchased. Since the factory Prius does not allow such alignment, the alignment kits are a plus. The springs, shocks, alignment kits are fairly simple to install. Eibach provided the progressive springs for the Prius that won in its class at the Bonneville Salt flats, over 130 mph!