i know that the honda guys who autox and road race run some REALLY high spring rates on one end or the other, but for street applications 11kg/mm front spring and 9kg/mm rear springs is going to be ridiculously stiff IMO
if you want to get you car VERY low and still be comfortable and safe, you're going to have to get a set of coilovers (not sleeve-overs) that have a thread shock body that can be adjusted up or down w/o adjusting the spring perch. by purchasing a set of coilovers that have this set up, it will enable you to lower your car considerably more yet still retain FULL suspension travel
here is a picture of the type of coilover i'm talking about (in case i didn't explain it that well)
Late,
Daniel