Not sure about spring rate, I'll be calling them on Monday about it as mine was a real pig to drive at spa in the wet, boost modulation was almost impossible.
Looking at their website again they say their uprated actuator (the one I and I assume you have) has a 'static break off pressure' of 12-14psi but this turbosmart one is 10psi. Theirs is not changeable where as this one is. I don't know what effect changing the spring would have?