Spring rates Nitron NTR Track Day

my Nitrons are being rebuild for Exige S specs.
On their website they recommend springs with a 450lbs rating front, and 600 rear.
Going much harder is not really what I had in mind, but what do you guys think is good ?
Will 425lbs springs give much more oversteer and 475lbs more understeer ?
My car is an Exige S PP with LSD.
tx !!!