When Sydneykid did the kits, he had the springs made esp to his spec's. They are all sold now and it was not worth him getting 10 sets (min) order done. He advised me the closest other springs available were the King springs.
I ended up fitting these with a set of his Bilstiens. My opinion the springs are more than heavy enough. They are around 25% heavy than stock at around 4kg. Springs are R33 GTR front, R32 GTR rears in Low spec. Do not use super low spec.
You mentioned you have kings already, but what shocks have you got?
If they are stock than I would say that is why you think the kings are too soft. Any spring8kg or higher on the street to stiff for our crappy roads.