I am not familiar with other brands than EPI...If I had a spring rate chart for other brands I would consider trying one...
You can't go wrong with an EPI blue secondary spring on a basically stock 06...one thing for sure is the stock spring is too soft if you are wanting real performance and especially if you have a cdi and sheave mod...both highly recommended...
An orange might work better...then again it might be stiffer than ideal for a stock motor...
The secondary spring sets up the dynamic range of the cvt...the ideal spring for all occassions imo is the one that gives you the highest top speed according to gps in 2wd low up against the rev limiter...
If acceleration on relatively level terrain is highest priority then you should select the one that yeilds the lowest e/t in a 0-300' holeshot...
It's going to vary depending on crankshaft hp, vehicle weight, tire size, and application...