after consulting one of my stock car design books. (sorry only speedway tech manuals in the house)
running the rear too soft will promote understeer. which i guess is a lot safer than oversteer(running the rear too hard)

I think you will find the car not as sharp in the corners but i assume that you want to do this for comfort reasons.

i would probably have a set of springs made and see how you like it. i wouldn't go out and buy new dampeners for the rear as its expensive and you wont see much difference between the two. a drop in spring rate by 3kg will be dramatic enough.

how is the car handling at the moment? lowering the rear spring rates you are going to dramatically increase the amount of understeer you experience while cornering.

i tried to dig around for info about HA spring rates but as they are no longer in the tein line up its hard to find any info about specs. i did find a reference to 10kg/mm front spring rates but I'm unsure if this was the standard.

as long as your happy with a dramatic increase in understeer and you don't track/drive too aggressively you probably wont have a problem.
you wont be able to hold as much speed as you used to while cornering. The car will just go straight ahead into the gutter/cliff face/guardrail.