That is a surprising spread front to rear. But remember that the front spring acts directly on the strut. The rear spring is set inward on the control arm. If I remember correctly, the wheel rate is about 60% of the spring rate in the rear and better than 90% in the front.
Still, I would have expected the front rate to be more like 250 lbs/in. I run 500 lb/in springs in the front of my e46.