Becareful when playing with stuff like this - can have some dramatic effects !!!
If it's really stiff handling you're after look to the overall package, not just the springs. The main need for custom rated springs is when the static weight of your car increase and you need something to cope with the extra load ie. VR6 Conversion.
These companies spend shed loads of time in R&D getting the springs rates set, so enjoy their hard work
I think you'll really need to get a quality set of shocks that are damper adjustable, uprated ARB's and dial that all in for whatever type of ride you're after .... much more flexible than a static set of springs you might not like too !