Very nice adjustable springs... but a little expensive when your target is just trackdays.
We use 2 cheap ways to make the front suspensions stiffer, first is very cheap and very efficient but not ajustable.
1) Buy 2 springs 10 euros each one !
Photos shows Bilstein but it works 10 time better with Koni :

Many choice there
http://www.geiss.fr/fr/produits/documents/RESSORTS.pdf
I have CXF4-089 321 N/mm
You need 2 parts machined which will do "guide" for springs

This one is usefull to but do nothing when braking,

Little and cheap springs do the same work than the RSR BAR even when you are braking
And the second way is....
Later thank you