Alternatively, instead of the Ground Control kits, you can get the HSD sleeve kits from Otomoto - http://otomoto.com.au/p/4116535/hsd-51mm-sleeve-kit-.html
 
This comes with all the required gear including helper springs, but you have the option of any spring rate you like from the below
 
3kg/mm (168lb/in)
4kg/mm (224lb/in)
5kg/mm (280lb/in)
6kg/mm (336lb/in)
7kg/mm (392lb/in)
8kg/mm (448lb/in)
9kg/mm (504lb/in)  
10kg/mm (560lb/in)
11kg/mm (616lb/in)
12kg/mm (672lb/in)
13kg/mm (728lb/in)
14kg/mm (784lb/in)
15kg/mm (840lb/in)
16kg/mm (896lb/in)
17kg/mm (952lb/in)
18kg/mm (1008lb/in)
 
If you find a particular rate is not what you like, you can buy a different rate spring for $120/pair.
I ran 3kg/6kg combo with Koni Yellow's and found it a nice, comfortable street setup which didn't bruise my kidneys. YMMV.