Springs are linear.
Why would you want progressive?
Linear springs give you always the same behaviour/forces no matter what compression/load.
Much better to tune/adjust the damper valving to, otherwise they will always be a compromise between the 'hard' and 'soft' part of the spring rates.
Springs are only 5 to 10% of the 'ride comfort' anyway, so it's much better to spend lots of time getting the damper valving spot on. Which is exactly what Lotus does with Bilstein.
I'm running 325/400 on my Nitron dampers and at the 'soft' setting on the dampers it rides just as comfirtable as the standard ones.
Will be going to 400/475 soon and have my dampers re-valved for that, but even that spring rate will be fine at the 'soft' setting on the dampers.
Crank the dampers up to 'hard' and the difference is night and day. Much, much bigger than the spring rate change itself.
Bye, Arno.