If he's running stiff springs all the way in then the effective spring rate would actually be quite low which seems to be verified with is driving experience so far. He could theoretically run a silver or black spring farther out on the arms and actually get a higher effective spring rate. Running a bronze all the way out would give the maximum rate possible and a black all the way in the minimum possible. Associated probably knows their product pretty well and probably chose those spring rates because they knew there was a combination(s) that would work well with their car and it's mounting options.

Has anyone figured out if any other springs from any other companies fit? The more options, the merrier.