It's only "too stiff" if softening makes the car faster with everything else constant.
Spring rate by itself doesn't tell you much - ride frequency is a more useful and universal number because it takes into consideration unsprung corner weight and the motion ratio of the spring. To start to make a guess how a car handles, you also need to know the front ride frequency for a comparison.
Camber looks ok for a dual purpose street/race car but for more optimal cornering I'd add .5-1 degree additional negative all around. Depends on what's suggested by your tire maker too - some (like bias-ply slicks) prescribe smaller amounts of negative camber for optimal performance.