Heaps of guys have done the upgrade from Teins, I get at least 5 enquiries a week. Every one so far has been ecstatic with the results, far better handling, more traction for acceleration and braking and a far superior ride. The guys who do circuit work have reported faster lap times.
So, the obvious question, why do you want to increase the spring rate? Springs are there to hold the car up and soak up the bumps, that's their job. Stabiliser bars control the roll. Anti dive geometry controls the dive under braking, anti squat geometry controls the squat under acceleration. The camber and caster adjuster increase the tyre contact pathc with the road/track. The shocks control the whole package.
Simply put, you don't need excessively high spring rates to have good handling.
cheers